To specify active atom regions the `active atoms`

section is required.

`selection type: [string]`

Keyword to give the type of atom selection. Required keyword.

Valid keyword values are:

`list`

A list of atoms will be given`range`

A range of atoms will be given`central atom`

A central atom and a radius will be given

`[method string]: [list, range or radius]`

Keyword to give the list, range or radius for an active space treated at the given level of theory.

Note

This keyword depends on the `selection type`

.

Valid method strings:

`hf`

`ccs`

`cc2`

`ccsd`

`cc3`

`ccsd(t)`

A list is specified using set notation: `{1,2,3}`

.

A range is specified as `[1,3]`

(equivalent to `{1,2,3}`

).

A radius is specified by a double precision real (e.g. `1.0d2`

) and is always in Angstrom units. The atoms within the radius \(r\) of the `central atom`

then define the active atoms.

Example: First three atoms in the input are chosen as HF active atoms space.

```
active atoms
selection type: list
hf: {1,2,3}
end active atoms
```

`central atom: [integer]`

Specifies the central atom in the active space. Requested for `selection type: central atom`

.

Note

This keyword is necessary in case of `selection type: central atom`

.

`inactive basis: [string]`

Specifies the basis set on the inactive atoms, that is the atoms not specified in the `active atoms`

section.
Optional.

Valid keyword values are basis sets available in \(e^T\).

`[method string] basis: [string]`

Set basis for active space treated at a given level of theory. Optional.

Valid method strings:

`hf`

`ccs`

`cc2`

`ccsd`

`cc3`

`ccsd(t)`

Valid keyword values are basis sets available in \(e^T\).

Example: First three atoms in the input are chosen as CCSD active atoms space with aug-cc-pVDZ basis.

```
active atoms
selection type: range
ccsd: [1,3]
ccsd basis: aug-cc-pVDZ
end active atoms
```

General coupled cluster keywords.

`Bath orbital`

Default: false

Add a bath orbital to the calculation with zero orbital energy and zero electron repulsion integrals.

Note

This keyword is required to compute ionized states.

The `cc mean value`

section is used to obtain CC ground state expectation values.

Note

This section is required if the keyword `mean value`

is given in the `do`

section.

`dipole`

Calculation of coupled cluster ground state dipole moment.

`quadrupole`

Calculation of coupled cluster ground state quadrupole moment.

Keywords specific to response calculations are given in the `cc response`

section.
Currently, the implemented methods are
EOM transition moments from the ground state and excited states
for CCS, CC2, CCSD, and CC3,
EOM polarizabilities for CCS, CC2, and CCSD,
and linear response transition moments from the ground state
and polarizabilities at the CCS level of theory.

Note

This section is required if the keyword `response`

is given in the `do`

section.

One of the keywords `polarizabilities`

, `permanent moments`

or `transition moments`

must be specified.

`polarizabilities`

Enables the calculation of polarizabilities.

`transition moments`

Enables the calculation of transition moments.

`eom`

Properties will be calculated within the equation of motion formalism.

Either this or the `lr`

keyword must be specified.

Available for CCS, CC2, CCSD, and CC3.

`lr`

Properties will be calculated within the linear response formalism.

Either this or the `eom`

keyword must be specified.

Available for CCS.

`dipole length`

Required keyword. Currently the only operator available for response calculations in \(e^T\).

`frequencies: {[real], [real], ...}`

Frequencies for which the polarizability shall be computed. Required for polarizabilities.

`initial states: {[real], [real], ...}`

Default: `{0}`

(Only the ground state is considered.)

Numbers of the states for which the transition/permanent moments shall be computed.

`permanent moments`

Enables the calculation of permanent moments.

Keywords related to time-dependent coupled cluster calculations go into the `cc td`

section.

Note

One of the keywords below must be specified if you have requested `time dependent state`

in the `do`

section.

`propagation`

Default: `false`

Perform real-time propagation of the coupled-cluster state.

`fft dipole moment`

Default: `false`

Perform complex fast Fourier transform of the dipole moment time series from an earlier real-time propagation calculation.

`fft electric field`

Default: `false`

Perform complex fast Fourier transform of the electric field time series from an earlier real-time propagation calculation.

The `do`

section is where the type of calculation is specified. It will determine the \(e^T\) engine used in the calculation.

Note

Only one of the keywords below has to be specified.
For example for the calculation of excited states only the keyword `excited state`

is required
even though the ground state equations have to be solved as well to obtain excited states.

`cholesky eri`

Keyword to run a Cholesky decomposition of the two-electron integrals. Note that this is done automatically for any coupled cluster calculation, the keyword should only be given if only Cholesky decomposition is to be performed.

`ground state`

Keyword to run a ground state calculation at the level of theory given in the `method`

section.
Enables the ground state or reference engine.

`ground state geoopt`

Keyword to run a Hartree-Fock ground state geometry optimization. Enables the ground state geometry optimization engine.

`mean value`

Keyword to calculate *coupled cluster* expectation values. Enables the mean value engine, which determines the coupled cluster ground state amplitudes and multipliers, and calculates the requested expectation value. Which mean value(s) to calculate are specified in the `cc mean value`

section.

Note

For Hartree-Fock calculations, one must write `ground state`

in `do`

and specify the mean value(s) to calculate in the `hf mean value`

section.

`excited state`

Keyword to run a coupled cluster excited state calculation. Enables the excited state engine, which calculates the ground and excited state amplitudes.

Note

The `cc es solver`

section is required for excited state calculations.

`response`

Keyword to enable the coupled cluster response engine. Implemented features are EOM transition moments and polarizabilities for CCS, CC2, CCSD and CC3, and LR transition moments and polarizabilities for CCS. The response engine drives the calculation of ground state amplitudes and multipliers, excited state vectors (left and right eigenvectors of the Jacobian matrix) and the requested property.

Note

The `cc response`

section is required for coupled cluster response calculations.

`restart`

Global restart keyword to activate restart where possible.

Note

eT will first check if restart is possible and use the default start guess if not.

`time dependent state`

Keyword to run coupled cluster time propagation. Enables the time-dependent engine.

Note

The `cc td`

section is required for coupled cluster time propagation.

To calculate the CCSD ground and four excited states, specify

```
do
excited state
end do
```

together with

```
method
hf
ccsd
end method
```

and

```
solver cc es
singlet states: 4
end solver cc es
```

Keywords related to the specification of electric field pulses for time-dependent coupled cluster calculations.

`envelope: {[integer],[integer],...}`

Specifies the envelopes to use for electric field pulse \(1,2,\ldots\).

Valid keyword values are:

`1`

Use Gaussian envelope.`2`

Use sine squared envelope.

`x polarization: {[real], [real], ...}`

\(x\) polarization for electric field pulse \(1,2,\ldots\).

`y polarization: {[real], [real], ...}`

\(y\) polarization for electric field pulse \(1,2,\ldots\).

`z polarization: {[real], [real], ...}`

\(z\) polarization for electric field pulse \(1,2,\ldots\).

`central time: {[real], [real], ...}`

Specifies the central time of electric field pulse \(1,2,\ldots\).

`width: {[real], [real], ...}`

Specifies the temporal widths of electric field pulse \(1,2,\ldots\) in atomic units. The width corresponds to the Gaussian root-mean-squared width of a pulse with a Gaussian envelope and the period of a pulse with a sine squared envelope.

`central angular frequency: {[real], [real], ...}`

Specifies the central angular frequencies of electric field pulse \(1,2,\ldots\) in atomic units.

`peak strength: {[real], [real], ...}`

Specifies the amplitude of the carrier wave of electric field pulse \(1,2,\ldots\) in atomic units.

`phase shift: {[real], [real], ...}`

Specifies the (carrier-envelope) phase shifts of electric field pulse \(1,2,\ldots\). The shift corresponds to the difference between the middle of the envelope and the maximum of the carrier wave.

`repetition: {[real], [real], ...}`

Default: `{1,1,...}`

(a single instance of each pulse)

Number of instances of electric field pulse \(1,2,\ldots\). The separation of the repeated pulses can be specified by the `separation`

keyword.

`separation: {[real], [real], ...}`

Number of repetitions (copies) of electric field pulse \(1,2,\ldots\).

Note

Should only be specified if the `repetition`

keyword has been specified.

The `frozen orbitals`

section is used to freeze orbitals for a reduced space coupled cluster calculation.

Orbitals which can be frozen:

- Core orbitals, for the frozen core approximation
- Hartree-Fock orbitals in the case where an active space is defined for the coupled cluster calculation.

`hf`

Default: false

This enables the freezing of Hartree-Fock orbitals after they are converged. Optional.

Note

This keyword depends on a coupled cluster `active atoms`

space being defined in the active atoms section.

`core`

Default: false

This enables the frozen core approximation. Optional.

Warning

This keyword can currently not be used if core excited states are to be calculated.

The geometry must be given as xyz coordinates either with Angstrom or Bohr units. The default units are Angstrom. The basis set is also given in the geometry section.

Minimal example for the geometry section

```
geometry
basis: aug-cc-pVDZ
H 0.86681 0.60144 5.00000
H -0.86681 0.60144 5.00000
O 0.00000 -0.07579 5.00000
end geometry
```

If other units than `Angstrom`

are desired,
this must be specified at the top of the geometry section with the `units`

keyword.
Possible values are `Angstrom`

and `Bohr`

.

```
geometry
units: Bohr
basis: aug-cc-pVDZ
H 1.63803 1.13655 9.44863
H -1.63803 1.13655 9.44863
O 0.00000 -0.14322 9.44863
end geometry
```

Different basis sets may be placed on the atoms using the `basis`

keyword.
An atom is given the last basis set specified above it,
e.g. in the following example the oxygen has the d-aug-cc-pVDZ basis
and the hydrogens have the aug-cc-pVDZ basis.
See here for a list of included basis sets.

```
geometry
basis: aug-cc-pVDZ
H 0.86681 0.60144 5.00000
H -0.86681 0.60144 5.00000
basis: d-aug-cc-pVDZ
O 0.00000 -0.07579 5.00000
end geometry
```

Basis functions can be placed without the corresponding atom by using the keyword
`ghost`

. Every atom specified after the keyword will have zero charge, which corresponds
to only placing the basis functions at the specified location. An H2 calculation run with the
same basis functions as H2O can be done using this geometry:

```
geometry
basis: aug-cc-pVDZ
H 0.86681 0.60144 5.00000
H -0.86681 0.60144 5.00000
ghost
O 0.00000 -0.07579 5.00000
end geometry
```

In case of QM/MM calculations,
the QM and MM portions have to be separated by a line containing `--`

.
The parameters for QM/MM calculations are placed after the XYZ coordinates.
In case of electrostatic QM/MM embedding (see molecular mechanics section),
the charge of each atom needs to be provided:

```
geometry
basis: cc-pVDZ
O 0.87273600 0.00000000 -1.24675400
H 0.28827300 0.00000000 -2.01085300
H 0.28827300 0.00000000 -0.48265500
--
O [IMol= 1] -0.77880300 0.00000000 1.13268300 [q=-0.834]
H [IMol= 1] -0.66668200 0.76409900 1.70629100 [q=+0.417]
H [IMol= 1] -0.66668200 -0.76409900 1.70629000 [q=+0.417]
end geometry
```

In case of polarizable QM/FQ (see molecular mechanics section), the electronegativities (chi) and chemical hardnesses (eta) are needed for each atom:

```
geometry
basis: cc-pVDZ
O 0.87273600 0.00000000 -1.24675400
H 0.28827300 0.00000000 -2.01085300
H 0.28827300 0.00000000 -0.48265500
--
O [IMol= 1] -0.77880300 0.00000000 1.13268300 [chi=0.11685879436,eta=0.58485173233]
H [IMol= 1] -0.66668200 0.76409900 1.70629100 [chi=0.00000000000,eta=0.62501048888]
H [IMol= 1] -0.66668200 -0.76409900 1.70629000 [chi=0.00000000000,eta=0.62501048888]
end geometry
```

The `hf mean value`

section is used to obtain HF expectation values.

`dipole`

Calculation of the HF dipole moment.

`quadrupole`

Calculation of the HF quadrupole moment.

In the integrals section you can specify settings for the handling of Cholesky vectors and electron repulsion integrals in coupled cluster calculations.

`cholesky storage: [string]`

Default: `memory`

if they take up less than 20% of the total memory; `disk`

otherwise

Specify how to store the Cholesky vectors. Optional.

Valid options are:

`memory`

Store Cholesky vectors in memory.`disk`

Store Cholesky vectors on file.

`eri storage: [string]`

Default: `memory`

if the entire matrix is needed in the calculation and they take up less than 20% of the total memory; `none`

otherwise

Specify how to store the electron repulsion integrals. Optional.

Valid options are:

`memory`

Store full electron repulsion matrix in memory.`none`

Always construct the integrals from Cholesky vectors.

Warning

`memory`

can slow down calculations for which the full integral matrix is not needed. This is the case for CCS and CC2 calculations. We recommend to use defaults.

`mo eri in memory`

Default: `false`

Override defaults and store full MO ERI matrix in memory if true. Recommended for time-dependent CC.

`t1 eri in memory`

Default: `false`

Override defaults and store full T1 ERI matrix in memory if true.

Keywords to specify the available memory is given in the `memory`

section.

`available: [integer]`

Default: 8

Specifies the available memory, default units are gigabytes. Optional.

`unit: [string]`

Default: GB

Specifies the units for the specified available memory. Optional.

Valid keyword values are:

- B
- KB
- MB
- GB

In the method section, the wavefunction method is specified. In the case of a post-HF calculation, both the reference wavefunction method and the post-HF method must be specified.

At the Hartree-Fock level of theory the following keywords may be specified:

`hf`

for restricted Hartree-Fock (RHF)`mlhf`

for multilevel Hartree-Fock`uhf`

for unrestricted Hartree-Fock

In coupled cluster calculations, either RHF or MLHF must be specified, in addition to the coupled cluster method.

The available coupled cluster methods are:

`ccs`

`cc2`

`lowmem-cc2`

`ccsd`

`ccsd(t)`

`cc3`

`mlcc2`

`mlccsd`

`mp2`

For a Hartree-Fock calculation, specify the type of wavefunction (`hf`

, `mlhf`

, or `uhf`

) in the method section.

```
method
hf
end method
```

To perform a coupled cluster calculation, both the coupled cluster method and the type of reference wavefunction must be specified.

```
method
hf
ccsd
end method
```

Keywords specific to multilevel coupled cluster enter into the `mlcc`

section.

`levels: [string], [string], ...`

Specifies the level of theory for the different active spaces. Required keyword.

Valid keyword values are:

`ccs`

`cc2`

`ccsd`

Note

Necessary for MLCCSD calculation.

`cc2 orbitals: [string]`

Specifies the type of orbitals used for orbital partitioning in MLCC2. Required keyword.

Valid keyword values are:

`cnto-approx`

Approximated correlated natural transition orbitals`cholesky`

Cholesky orbitals (occupied and virtual)`cholesky-pao`

Cholesky occupied orbitals and projected atomic orbitals for the virtuals.`nto-canonical`

Natural transition orbitals for occupied and canonical for virtuals.

Warning

`nto-canonical`

and `cholesky`

orbitals are not generally recommended.

`ccsd orbitals: [string]`

Specifies the type of orbitals used for orbital partitioning in MLCCSD. Required keyword.

Valid keyword values are:

`cnto-approx`

Approximated correlated natural transition orbitals`cholesky`

Cholesky orbitals (occupied and virtual)`cholesky-pao`

Cholesky occupied orbitals and projected atomic orbitals for the virtuals.`nto-canonical`

Natural transition orbitals for occupied and canonical for virtuals.

Warning

`nto-canonical`

and `cholesky`

orbitals are not generally recommended.

`cholesky threshold: [real]`

Default: \(1.0\cdot 10^{-2}\) (or 1.0d-2)

Threshold for the Cholesky decomposition of the density. Only used with `cc2 orbitals: cholesky`

or `cc2 orbitals: cholesky-pao`

`cnto occupied cc2: [integer]`

Determines the number of occupied orbitals in the CC2 orbital space for the MLCC2 calculation.

Note

Necessary if `cc2 orbitals: cnto-approx`

is given.

`cnto virtual cc2: [integer]`

Default: \(n_v^{\text{CC2}} = n_o^{\text{CC2}}\cdot\frac{n_v}{n_o}\)

Sets the number of virtual orbitals in the CC2 orbital space for the MLCC2 calculation. Optional.

Note

Only used if `cc2 orbitals: cnto-approx`

is given.

`cnto occupied ccsd: [integer]`

Determines the number of occupied orbitals in the CCSD orbital space for the MLCCSD calculation.

Note

Necessary if `ccsd orbitals: cnto-approx`

is given.

`cnto virtual ccsd: [integer]`

Default: \(n_v^{\text{CCSD}} = n_o^{\text{CCSD}}\cdot\frac{n_v}{n_o}\)

Sets the number of virtual orbitals in the CCSD orbital space for the MLCCSD calculation. Optional.

Note

Only used if `ccsd orbitals: cnto-approx`

is given.

`cnto states: {[integer], [integer], ...}`

Determines which CCS excited states are used to construct the approximated CNTOs.

Note

Necessary if `cc2 orbitals: cnto-approx`

or `ccsd orbitals: cnto-approx`

is given.

`print ccs calculation`

Default: false

Enables the printing of the CCS calculation in the case of `cc2 orbitals: cnto-approx`

or `ccsd orbitals: cnto-approx`

. Optional.

`print cc2 calculation`

Default: false

Enables the printing of the CC2 calculation in the case of `ccsd orbitals: cnto`

. Optional.

`nto occupied cc2: [integer]`

Determines the number of occupied orbitals in the CC2 orbital space for the MLCC2 calculation.

Note

Necessary if `cc2 orbitals: nto-canonical`

is given.

`nto occupied ccsd: [integer]`

Determines the number of occupied orbitals in the CCSD orbital space for the MLCCSD calculation.

Note

Necessary if `ccsd orbitals: nto-canonical`

is given.

`nto virtual cc2: [integer]`

Determines the number of virtual orbitals in the CC2 orbital space for the MLCC2 calculation. Optional.

Note

Only used if `cc2 orbitals: nto-canonical`

is given.

`canonical virtual cc2: [integer]`

Default: \(n_v^{\text{CC2}} = n_o^{\text{CC2}}\cdot\frac{n_v}{n_o}\)

Sets the number of virtual orbitals in the CC2 orbital space for the MLCC2 calculation. Optional.

Note

Only used if `cc2 orbitals: nto-canonical`

is given.

`canonical virtual ccsd: [integer]`

Default: \(n_v^{\text{CCSD}} = n_o^{\text{CCSD}}\cdot\frac{n_v}{n_o}\)

Sets the number of virtual orbitals in the CCSD orbital space for the MLCCSD calculation. Optional.

Note

Only used if `ccsd orbitals: nto-canonical`

is given.

`nto states: {[integer], [integer], ...}`

Determines which CCS excited states are used to construct the NTOs.

Note

Necessary if `cc2 orbitals: nto-canonical`

is given.

`cnto restart`

Default: false

If specified, the solver will attempt to restart from previously determined CNTO matrices (M and N). Optional.

`orbital restart`

Default: false

If specified, orbital partitioning is skipped and MLCC orbitals (and sizes of orbital sets) are read from file. Optional.

MM specific keywords enter into the `molecular mechanics`

section.

`forcefield: [string]`

Default: `non-polarizable`

Specifies the forcefield to be used for the MM portion.

Valid keyword values are:

`non-polarizable`

Electrostatic QM/MM EmbeddingNote

The charge has to be provided for each atom in the geometry section.

`fq`

Fluctuating Charge force field.Note

The electronegativity and chemical hardness has to be provided for each atom in the geometry section.

`algorithm: [string]`

Default: `mat_inversion`

Selects the algorithm to be used to solve the FQ equation. So far only the inversion algorithm is implemented. Optional.

Keywords specific to multilevel Hartree-Fock enter into the `multilevel hf`

section.

`initial hf optimization`

Default: `false`

Enables an initial optimization of the full density through a standard HF calculation to a low threshold.

`initial hf threshold: [real]`

Default: \(1.0\cdot10^{-1}\) (or 1.0d-1)

Threshold for the initial HF optimization. Should only be specified if `initial hf optimization`

is also specified.

`print initial hf`

Default: false

Enables the printing of the initial HF optimization in the case that `initial hf optimization`

is given. Optional.

`cholesky virtuals`

Default: `false`

Enable the use of Cholesky decomposition of the virtual AO density to construct the active virtual orbitals. The default is to use projected atomic orbitals.

`cholesky threshold: [real]`

Default: \(1.0\cdot10^{-2}\) or (1.0d-2)

Threshold for the Cholesky decomposition of the AO density to construct an active space.

`project on minimal basis`

Default: `false`

First diagonalization of the AO fock matrix will be performed in a minimal basis.

Keywords specific to the polarizable continuum model enter into the `pcm`

section.

`input: [string]`

Specifies if the input parameters are given in the \(e^{T}\) input file or in an external file handled directly by the external library PCMSolver. Required.

Valid keyword values are

`external`

Note

No further input has to be given to

`pcm`

because the parameters have to be specified in an external*.pcm*-file.

`internal`

`tesserae area: [real]`

Default: 0.3

Area of the finite elements (*tesserae*) the surface is constructed from given in square Angstrom.

`solver type: [string]`

Default: `iefpcm`

Selects the type of solver to be used to solve the PCM equation.

Valid keyword values are

`iefpcm`

Integral Equation Formalism PCM`cpcm`

conductor-like PCM

In this section you can specify settings related to the main output files.

`output print level: [string]`

Specifies the print level for the main output file. Default is `normal`

. Optional.

Valid keyword values are:

`minimal`

Only banners, final results like total energies or excitation energies, solver settings, and other essential information.`normal`

In addition to minimal, print iteration information and details like orbital energies, amplitude analysis, and other non-essential information.`verbose`

In addition to normal, print all relevant information for users. This can make the output difficult to read and navigate.`debug`

In addition to verbose, prints information mostly relevant for debugging code that behaves unexpectedly.

`timing print level: [string]`

Specifies the print level for the timing file. Default is `normal`

. Optional.

Valid keyword values are:

`minimal`

Total solver timings, total program time, and other essential timings.`normal`

In addition to minimal, iteration times and details like time to calculate omega, the Fock matrix, and other expensive terms.`verbose`

In addition to normal, times for subtasks, such as micro-iteration times as well as individual contributions to the omega vector.`debug`

In addition to verbose, prints timings mostly relevant for debugging code that behaves unexpectedly.

`full references`

If specified, implementation references will be printed in APA style. Otherwise, only the DOIs are printed to the output file.

`z-matrix`

If specified, prints the z-matrix to the output file.

Keywords related to solving the excited state coupled cluster equations go into the `solver cc es`

section.
Required for calculations of excited states!

`singlet states: [integer]`

Specifies the number of singlet excited states to calculate.

`algorithm: [string]`

Default: `davidson`

for CCS, CC2, MLCC2, and CCSD; `non-linear davidson`

for lowmem-CC2 and CC3.

Solver to use for converging the excited state equations.

Valid keyword values are:

`davidson`

Use Davidson algorithm with residuals preconditioned with the orbital differences approximation of the Jacobian. Cannot be used for lowmem-CC2 and CC3.`diis`

Use DIIS algorithm with update estimates obtained from the orbital differences approximation of the Jacobian. Can be used for lowmem-CC2 and CC3.`non-linear davidson`

Use the non-linear Davidson algorithm with residuals preconditioned with the orbital differences approximation of the Jacobian. Can be used for lowmem-CC2 and CC3.`asymmetric lanczos`

Use the asymmetric Lanczos algorithm for excitation energies. EOM oscillator strengths will be calculated as well. Cannot be used for lowmem-CC2, MLCC2, and CC3.

`chain length: [integer]`

Specifies the dimension of the reduced (Krylov sub-) space for the asymmetric Lanczos algorithm. Required for `algorithm: asymmetric lanczos`

.

`right eigenvectors`

Default: true

If specified, solve for the right eigenvectors of the Jacobian matrix. This keyword should only be specified for an excited state calculation. For property calculations, the program will solve for both left and right vectors. Optional.

Note

For response calculations or when using the asymmetric lanczos algorithm, this keyword is ignored.

`left eigenvectors`

Default: false

If specified, solve for the left eigenvectors of the Jacobian matrix. This keyword should only be specified for an excited state calculation. For property calculations, the program will solve for both left and right vectors. Optional.

Note

For response calculations or when using the asymmetric lanczos algorithm, this keyword is ignored.

`energy threshold: [real]`

Default: \(10^{-3}\) (or 1.0d-3)

Energy convergence threshold, as measured with respect to the previous iteration. Optional.

Note

The solvers will not check for the energy convergence, if the energy threshold is not set.

`residual threshold: [real]`

Default: \(10^{-3}\) (or 1.0d-3)

Threshold of the \(L^{2}\)-norm of the residual vector of the excited state equation. Optional

`core excitation: { integer, integer, ... }`

Default: false

Solve for core excitations within the CVS approximation. The integers specify which orbitals to excite out of. Orbitals are ordered according to orbital energy (canonical orbitals). If the keyword has been specified, CVS will be activated automatically. Optional.

`remove core: { integer, integer, ... }`

Default: false

Valence excitations, but with excitations from core MOs projected out. The integers specify which core orbitals from which one should not excite. Orbitals are ordered according to orbital energy (canonical orbitals). Optional.

Note

This is the orthogonal projection to `core excitation: { integer, integer, ... }`

Warning

Cannot be used in CVS calculations or when frozen core is enabled.

`ionization`

Default: false

Solve for ionized state. If this keyword is specified, the ionized state will be calculated using a bath orbital and projection similar to CVS. Optional.

Note

For ionizations a bath orbital has to be requested in the cc section

`max iterations: [integer]`

Default: 100

The maximum number of iterations. The solver stops if the number of iterations exceeds this number. Optional.

`diis dimension: [integer]`

Default: 20

Number of previous DIIS records to keep. Optional.

Note

Only relevant for `algorithm: diis`

.

`restart`

Default: false

If specified, the solver will attempt to restart from a previous calculation. Optional.

`storage: [string]`

Default: `disk`

Selects storage of excited state records. Optional.

Valid keyword values are:

`memory`

Stores DIIS records in memory.`disk`

Stores DIIS records on file.

`crop`

Default: false

If specified, the CROP version of DIIS will be enabled. Optional.

Note

Only relevant for `algorithm: diis`

.

`max reduced dimension: [integer]`

Default: 100

The maximal dimension of the reduced space of the Davidson procedure. Optional.

Note

Only relevant for `algorithm: davidson`

.

`lanczos normalization: [integer]`

Default: `asymmetric`

Specifies the type of biorthonormalization for the asymmetric Lanczos algorithm. Optional.

Valid keyword values are:

`asymmetric`

Which enables \(\tilde{p} = p\) and \(\tilde{q} = \frac{q}{p\cdot q}\)`symmetric`

Which enables \(\tilde{p} = \frac{p}{\sqrt{|p\cdot q|}}\) and \(\tilde{q} = \text{sgn}(p\cdot q)\frac{q}{\sqrt{|p\cdot q|}}\)

`max micro iterations: [integer]`

Default: \(100\)

Maximum number of iterations in the non-linear Davidson solver. Optional.

`davidson preconvergence`

Performs pre-iterations with non-linear Davidson in the DIIS solver.

`preconvergence threshold: [real]`

Default: \(10^{-2}\) (or 1.0d-2)

Threshold for pre-iterations with non-linear Davidson in the DIIS solver.

`max micro iterations: [integer]`

Default: \(100\)

Maximum number of micro iterations in the non-linear Davidson solver.

`rel micro threshold: [real]`

Default: \(10^{-1}\) (or 1.0d-1)

Threshold for convergence in the micro iterations of the non-linear Davidson solver. This threshold is relative to the current norm of the residuals.

Keywords related to solving the ground state coupled cluster equations go into the `solver cc gs`

section. This section is optional. If it is not given, defaults will be used for all keywords.

`algorithm: [string]`

Default: `diis`

Solver to use for converging the amplitude equations. Optional.

Valid keyword values are:

`diis`

Quasi-Newton algorithm accelerated by DIIS. Uses the orbital differences approximation of the coupled cluster Jacobian.`newton-raphson`

. Newton algorithm accelerated by DIIS.

`energy threshold: [real]`

Default: \(10^{-5}\) (or 1.0d-5)

Energy convergence threshold, as measured with respect to the previous iteration. Optional.

Note

The solvers will not check for the energy convergence, if the energy threshold is not set.

`omega threshold: [real]`

Default: \(10^{-5}\) (or 1.0d-5)

Threshold of the \(L^{2}\)-norm of the amplitude equations vector \(\Omega_\mu = \langle \mu \vert \bar{H} \vert \text{HF} \rangle\). Optional.

`multimodel newton`

Default: false

If specified, the Newton-Raphson algorithm will solve the micro-iterations using a lower-level approximation of the Jacobian matrix. Must be combined with `algorithm: newton-raphson`

. Optional.

`crop`

Default: false

If specified, the CROP version of DIIS will be enabled. Optional.

`max iterations: [integer]`

Default: 100

The maximum number of iterations. The solver stops if the number of iterations exceeds this number. Optional.

`max micro iterations: [integer]`

Default: 100

The maximum number of micro-iterations in the exact Newton solver (see `algorithm: newton-raphson`

).
The solver stops if the number of iterations exceeds this number.
Optional.

`rel micro iterations: [real]`

Default: \(10^{-2}\) (or 1.0d-2)

The relative threshold \(\tau\) used for micro-iterations by the exact Newton solver (see `algorithm: newton-raphson`

). The micro-iterations are considered converged if the \(L^{2}\)-norm of the Newton equation is less than \(\tau \vert\vert \boldsymbol{\Omega} \vert\vert\). Optional.

`storage: [string]`

Default: `disk`

Selects storage of DIIS records in coupled cluster calculations. Optional.

Valid keyword values are:

`memory`

Stores DIIS records in memory.`disk`

Stores DIIS records on file.

`micro iteration storage: [string]`

Default: value of the keyword storage

Selects storage of records in the micro iterations (if any) in ground state coupled cluster calculations. Optional.

Valid keyword values are:

`memory`

Stores records in memory.`disk`

Stores records on file.

`diis dimension: [integer]`

Default: 8

Number of previous DIIS records to keep. Optional.

`restart`

Default: false

If specified, the solver will attempt to restart from a previous calculation. Optional.

Keywords related to solving the coupled cluster multiplier equation (left coupled cluster ground state) go into this section.

`algorithm: [string]`

Default: `diis`

for `ccs`

, `lowmem-cc2`

, `cc2`

, and `cc3`

; `davidson`

for other coupled cluster models.

Solver to use for converging the multiplier equation. Not supported for MLCC2.

Valid keyword values are:

`davidson`

Use Davidson algorithm with residuals preconditioned with orbital differences approximation of the Jacobian. Cannot currently be used for lowmem-CC2, CC2, and CC3.`diis`

Use DIIS algorithm with update estimates obtained from the orbital differences approximation of the Jacobian. Must be used for lowmem-CC2, CC2, and CC3.

`threshold: [real]`

Default: \(10^{-5}\) (or 1.0d-5)

Threshold of the \(L^{2}\)-norm of the residual vector of the multiplier equation. Optional.

`max iterations: [integer]`

Default: 100

The maximum number of iterations. The solver stops if the number of iterations exceeds this number. Optional.

`diis dimension: [integer]`

Default: 20

Number of previous DIIS records to keep. Optional.

Note

Only relevant for `algorithm: diis`

.

`restart`

Default: false

If specified, the solver will attempt to restart from a previous calculation. Optional.

`storage: [string]`

Default: `disk`

Selects storage of solver subspace records. Optional.

Valid keyword values are:

`memory`

Stores records in memory.`disk`

Stores records on file.

`crop`

Default: false

If specified, the CROP version of DIIS will be enabled. Optional.

Note

Only relevant for `algorithm: diis`

.

`max reduced dimension: [integer]`

Default: 100

The maximal dimension of the reduced space of the Davidson procedure. Optional.

Note

Only relevant for `algorithm: davidson`

.

Keywords related to time-dependent coupled cluster propagation settings.

`integrator: [string]`

Specifies the integration method used for real-time propagation. Required.

Valid keyword values are:

`rk4`

Fourth-order Runge-Kutta (RK4)`gl2`

Second-order Gauss-Legendre (GL2)`gl4`

Fourth-order Gauss-Legendre (GL4)`gl6`

Sixth-order Gauss-Legendre (GL6)

`initial time: [real]`

The start time for the real-time propagation given in atomic units. Required.

`final time: [real]`

The end time for the real-time propagation given in atomic units. Required.

`time step: [real]`

Size of the time steps for the real-time propagation given in atomic units. Required.

`steps between output: [integer]`

Default: \(1\)

Specifies how many time steps the solver should take between each time output (energy, dipole moment, …) is written to file. Optional.

`implicit treshold: [real]`

Default: \(10^{-11}\) (or 1.0d-11)

Specifies how tightly the Euclidian norm of the residual of implicit Runge-Kutta methods should converge before going to the next time step. Optional.

`energy output`

Default: `false`

Write energy to file every `steps between output`

. Optional.

`dipole moment output`

Default: `false`

Write dipole moment to file every `steps between output`

. Optional.

`electric field output`

Default: `false`

Write electric field to file every `steps between output`

. Optional.

`amplitudes output`

Default: `false`

Write cluster amplitudes to file every `steps between output`

. Optional.

`multipliers output`

Default: `false`

Write multipliers to file every `steps between output`

. Optional.

`density matrix output`

Default: `false`

Write molecular orbital (MO) density matrix to file every `steps between output`

. Optional.

Keywords related to solving the coupled cluster response equations. Used when solving the amplitude response and multiplier response equations.

`threshold: [real]`

Default: \(10^{-3}\) (or 1.0d-3)

Residual norm threshold for convergence. Optional.

`storage: [string]`

Default: `disk`

Storage for Davidson records. Optional.

Valid options are:

`disk`

Store records on file.`memory`

Store records in memory.

`max iterations: [real]`

Default: \(100\)

Keywords related to the Cholesky decomposition of electron repulsion integrals go into the `solver cholesky`

section. This section is optional. If it is not given, defaults will be used for all keywords.

`threshold: [real]`

Default: \(10^{-4}\) (or 1.0d-4).

Decomposition threshold. All electron repulsion integrals will be reproduced by the Cholesky vectors to within this threshold. This threshold puts an upper limit to the accuracy of coupled cluster calculations. Optional.

`batches: [integer]`

Default: 1

Number of batches
\(n_b\)
in partitioned Cholesky decomposition. In this procedure, the Cholesky basis is chosen from a reduced set generated by decomposing diagonal blocks of the matrix (of which there are
\(n_b\)).
Introduces an error of about an order of magnitude higher than `threshold`

. Optional.

`one center`

Default: false

If specified, the Cholesky decomposition is restricted to diagonal elements
\(g_{\alpha\beta,\alpha\beta}\)
for which both atomic orbital indices,
\(\alpha\)
and
\(\beta\)
, are centered on the same atom. Introduces an error of about
\(10^{-3}\)
that cannot be reduced further by `threshold`

. Optional.

`span: [real]`

Default: \(10^{-2}\) (or 1.0d-2)

Span factor \(\sigma\) . For a given iteration of the Cholesky decomposition, this number determines the range of possible pivots to select/qualify: \(D_{\alpha\beta} \geq \sigma D_\mathrm{max}\) . Optional.

`qualified: [integer]`

Default: \(1000\)

Maximum number of qualified pivots in an iteration of the Cholesky decomposition. Optional.

`mo screening`

Perform Cholesky decomposition such that the MO electron repulsion integrals are targeted. The default screening targets the AO integrals.

Note

This keyword should always be used for CC-in-HF and CC-in-MLHF calculations, for which the the number of MOs is typically much smaller than the number of AOs.

Keywords related to Fast Fourier transform (FFT) functionality for the time-dependent coupled cluster code. Below you find keywords that are valid in the two sections `solver fft dipole moment`

and `solver fft electric field`

.

Note

These sections are relevant only if you have specified `fft dipole moment`

or `fft electric field`

in the `cc td`

section.

`initial time: [real]`

Specifies the start of the interval of interest in the time series file to Fourier transform. Required.

`final time: [real]`

Specifies the end of the interval of interest in the time series file to Fourier transform. Required.

`time step: [real]`

Specifies the time step between the data points in the time series file to Fourier transform. Required.

Warning

This must be equal to \(\text{time step} \times \text{steps between output}\) in the propagation section of the calculation that generated time series file.

Keywords related to the SCF solver go into the `solver scf`

section.
This section is optional. If it is not given, defaults will be used for all keywords.

`algorithm: [string]`

Default: `scf-diis`

Selects the solver to use.

Valid keyword values are:

`scf-diis`

AO-based self-consistent Roothan-Hall algorithm with direct inversion of the iterative subspace (DIIS) acceleration.`scf`

Self-consistent Roothan-Hall algorithm.`mo-scf-diis`

MO-based self-consistent Roothan-Hall algorithm with DIIS acceleration. Recommended for multilevel Hartree-Fock (MLHF).

`energy threshold: [real]`

Default: \(10^{-7}\) (or 1.0d-7)

Energy convergence threshold, as measured with respect to the previous iteration.

Note

The solvers will not check for the energy convergence, if the energy threshold is not set.

`gradient threshold: [real]`

Default: \(10^{-7}\) (or 1.0d-7)

Gradient threshold \(\tau\). The equations have converged if \(\max \mathbf{G} < \tau\).

`storage: [string]`

Default: `memory`

Selects storage of DIIS records.

Valid keyword values are:

`memory`

Stores DIIS records in memory.`disk`

Stores DIIS records on file.

`print orbitals`

Default: `false`

If specified, the *mo_coefficients.out file is created and copied to the working directory by `eT_launch`

.

`write molden`

Default: `false`

If specified, a molden input file called *.molden is created and copied to the working directory by `eT_launch`

.

`crop`

Default: `false`

If specified, the conjugate residual with optimal trial vectors (CROP) version of DIIS will be enabled.

`cumulative fock threshold: [real]`

Default:
\(1.0\) (or *1.0d0*)

When the gradient max-norm reaches this threshold, the Fock matrix is built using the difference in the density matrix relative to the previous iteration. When the max-norm exceeds the threshold, the Fock matrix is built directly using the current density matrix.

`max iterations: [integer]`

Default: \(100\)

The maximum number of iterations. The solver stops if the number of iterations exceeds this number.

`diis dimension: [integer]`

Default: \(8\)

Number of previous DIIS records to keep.

`restart`

Default: `false`

If specified, the solver will attempt to restart from a previous calculation.

`skip`

Default: `false`

If specified, orbitals will be read from file, the convergence of the gradient will be checked, but the rest of the SCF solver will be skipped.

Note

This is used to restart from eT v1.0.x, as the orbitals will not be flipped.

`ao density guess: [string]`

Default: `sad`

Which atomic orbital density matrix to use as start guess.

Valid keyword values are:

`sad`

Use the superposition of atomic densities (SAD) guess. This is built on-the-fly by performing spherically averaged UHF calculations on each unique atom (and basis) in the specified molecular system.`core`

Use the density obtained by approximating the Fock matrix by its one-electron contribution and performing one Fock diagonalization.

`coulomb threshold: [real]`

Default: \(10^{-6}*(\text{gradient threshold})\)

The threshold for neglecting Coulomb contributions to the two-electron part of the AO Fock matrix.

`exchange threshold: [real]`

Default: \(10^{-4}*(\text{gradient threshold})\)

The threshold for neglecting exchange contributions to the two-electron part of the AO Fock matrix. Must be higher than `coulomb threshold`

. If this is specified with a lower value than `coulomb threshold`

, it will be set equal to `coulomb threshold`

. See the output.

`integral precision: [real]`

Default: \(\text{(coulomb threshold)}^2\)

The \(\epsilon\) value for Libint 2. Gives the precision in the electron repulsion integrals.

Note

Changes dynamically during Fock construction to give the required precision in the Fock matrix and not the integrals (small density contributions require less accuracy in the integrals).

Warning

The value does not guarantee the given precision. It is highly recommended to let the program handle the integral precision value.

`integral cutoff: [real]`

Default: \(\text{(coulomb threshold)}\)

Shell-pairs for which all the electron repulsion integrals are smaller than this value are neglected in the Fock matrix construction.

`one-electron integral cutoff: [real]`

Default: \(\text{(coulomb threshold)}\cdot10^{-5}\)

One-electron Hamiltonian integrals are neglected for a shell-pair for which all the overlap integrals are smaller than this value.

Keywords related to the HF geometry optimization solver go in here.

Warning

We currently do not recommend using the program for geometry optimization. The solver is known to be ineffective for systems larger than a few atoms.

`algorithm: [string]`

Default: `bfgs`

Selects the solver to use. Optional.

Valid keyword values are:

`bfgs`

A Broyden-Fletcher-Goldberg-Shanno (BFGS) solver using cartesian coordinates and a rational function (RF) level shift obtained from an augmented Hessian.

`max step: [real]`

Default:
\(0.5\) (or *0.5d0*)

Maximum accepted step length in \(L^{2}\)-norm. Rescales the step to the boundary otherwise. Optional.

`energy threshold: [real]`

Default: \(10^{-4}\) (or 1.0d-4)

Energy convergence threshold, as measured with respect to the previous iteration. Optional.

`gradient threshold: [real]`

Default: \(10^{-4}\) (or 1.0d-4)

Threshold of the \(L^{2}\)-norm of the energy gradient. Optional.

`max iterations: [integer]`

Default: 100

`restart`

Default: false

If specified, the solver will attempt to restart from a previous calculation. Optional.

In the system section of the input the name, charge, and multiplicity of the system is given. Furthermore, the use of cartesian Gaussians may be specified in this section.

`name: [string]`

Default: none

Specifies the name of the calculation, which is printed in the output file.

`charge: [integer]`

Default: \(0\)

Specifies the charge of the system.

`multiplicity: [integer]`

Default: \(1\)

Specifies the spin multiplicity of the system.

`cartesian gaussians`

Enforce cartesian Gaussians basis functions for all atoms. Default for Pople basis sets.

`pure gaussians`

Enforce spherical Gaussian basis functions for all atoms. Default for all basis sets except Pople basis sets.

A minimal example of the *system* section, includes only the name of the calculation. This can be any string, e.g.,

```
system
name: water
end system
```

The charge and multiplicity is given in the example below. Additionally, cartesian Gaussians are enabled.

```
system
name: water
charge: 0
multiplicity: 1
cartesian gaussians
end system
```

The `visualization`

section is used for plotting orbitals and densities.

`file format: [string]`

Default: `plt`

Specify format of the output files containing the grid data. Optional.

Valid options are:

`plt`

`cube`

`grid buffer: [real]`

Default: \(2.0\) (or 2.0d0)

Sets the distance between the edge of the grid (x, y, and z direction) and the molecule in Angstrom units. Optional.

`grid max: {[real], [real], [real]}`

Sets the maximum values of the grid in x, y and z direction in Angstrom. Optional.

Note

`grid min`

is required if `grid max`

is specified.

`grid buffer`

is not used in this case.

`grid min: {[real], [real], [real]}`

Sets the minimum values of the grid in x, y and z direction in Angstrom. Optional.

Note

`grid max`

is required if `grid min`

is specified.

`grid buffer`

is not used in this case.

`grid spacing: [real]`

Default: \(0.1\) (or 1.0d-1)

Sets the spacing between grid points for the visualization given in Angstrom units. Optional.

`plot cc density`

Plots the coupled cluster density. Optional.

Note

This keyword is only read if `cc mean value`

or `response`

is specified in the `do`

section.

`plot es densities`

Plots the coupled cluster excited state densities for the `states to plot`

. Optional.

Note

This keyword is only read if `response`

is specified in the `do`

section.

`plot hf active density`

Plots the active HF density in the case of a reduced space calculation. Optional.

Note

This keyword is only read if `hf`

is specified in the `frozen orbitals`

section.

`plot hf density`

Plots the HF density. Optional.

`plot hf orbitals: {[integer], [integer], ...}`

Plots the canonical orbitals given in the comma separated list. Optional.

`plot transition densities`

Plots the coupled cluster transition densities for the `states to plot`

. Optional.

Note

This keyword is only read if `response`

is specified in the `do`

section.

`states to plot: {[integer], [integer], ...}`

List of integers specifying which densities should be plotted.

Note

By default the densities of the states specified in `initial states`

from the response section are plotted.