# List of all sections and keywords¶

## active atoms¶

To specify active atom regions the active atoms section is required.

### Keywords¶

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


## cc mean value¶

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.

### Keywords¶

dipole

Calculation of coupled cluster ground state dipole moment.

quadrupole

Calculation of coupled cluster ground state quadrupole moment.

## cc response¶

Keywords specific to response calculations are given in the cc response section. This currently involves EOM transition moments and polarizabilities for CCS, CC2, CCSD, and CC3, and linear response transition moments and polarizabilities at the CCS level of theory.

Note

This section is required if the keyword response is given in the do section.

### Keywords¶

polarizabilities

Enables the calculation of polarizabilities. Either this or the transition moments keyword must be specified.

transition moments

Enables the calculation of transition moments. Either this or the polarizabilities keyword must be specified.

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.

## cc td¶

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.

### Keywords¶

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.

## do¶

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.

### Keywords¶

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.

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.

### Example¶

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


## electric field¶

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

### Required keywords¶

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.

### Optional keywords¶

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.

## frozen orbitals¶

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.

### Keywords¶

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.

## geometry¶

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


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


## hf mean value¶

The hf mean value section is used to obtain HF expectation values.

### Keywords¶

dipole

Calculation of the HF dipole moment.

quadrupole

Calculation of the HF quadrupole moment.

## integrals¶

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

### Keywords¶

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.

## memory¶

Keywords to specify the available memory is given in the memory section.

### Keywords¶

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

• MB

• GB

## method¶

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.

### Hartree-Fock methods¶

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.

### Coupled cluster methods¶

The available coupled cluster methods are:

• ccs

• cc2

• lowmem-cc2

• ccsd

• ccsd(t)

• cc3

• mlcc2

### Other methods¶

• mp2

### Examples¶

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


## mlcc¶

Keywords specific to multilevel coupled cluster enter into the mlcc section.

### Keywords¶

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.

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

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

Note

Necessary if cc2 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. 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 virtual cc2: [integer]

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

Note

Necessary 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.

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

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

Note

Necessary if cc2 orbitals: nto-canonical is given.

## molecular mechanics¶

MM specific keywords enter into the molecular mechanics section.

### Keywords¶

forcefield: [string]

Default: non-polarizable

Specifies the forcefield to be used for the MM portion.

Valid keyword values are:

• non-polarizable Electrostatic QM/MM Embedding

Note

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.

## multilevel hf¶

Keywords specific to multilevel Hartree-Fock enter into the multilevel hf section.

### Keywords¶

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.

## pcm¶

Keywords specific to the polarizable continuum model enter into the pcm section.

### Keywords¶

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

solvent: [string]

Specifies the solvent outside the cavity.

Valid keyword values

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

## print¶

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

### Keywords¶

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.

## solver cc es¶

Keywords related to solving the excited state coupled cluster equations go into the solver cc es section. Required for calculations of excited states!

### Required keywords¶

singlet states: [integer]

Specifies the number of singlet excited states to calculate.

algorithm: [string]

Default: davidson for CCS, CC2, MLCC2, and CCSD; diis 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. Must be used for lowmem-CC2 and CC3; cannot be used for MLCC2.

• 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.

### Optional keywords¶

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: $$10^{-6}$$ (or 1.0d-6)

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

residual threshold: [real]

Default: $$10^{-6}$$ (or 1.0d-6)

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.

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.

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 $$\tilde{q} = \text{sgn}(p\cdot q)\frac{q}{\sqrt{|p\cdot q|}}$$

## solver cc gs¶

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.

### Keywords¶

algorithm: [string]

Default: diis

Solver to use for converging the amplitude equations. Optional.

Valid keyword values are:

• diis Inexact 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: $$10^{-6}$$ (or 1.0d-6)

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

omega threshold: [real]

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

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.

## solver cc multipliers¶

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

### Keywords¶

algorithm: [string]

Default: diis for 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: $$10^{-6}$$ (or 1.0d-6)

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.

## solver cc propagation¶

Keywords related to time-dependent coupled cluster propagation settings.

### Required keywords¶

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.

### Optional keywords¶

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.

## solver cc response¶

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

### Keywords¶

threshold: [real]

Default: $$10^{-6}$$ (or 1.0d-6)

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: $$100$$

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

## solver cholesky¶

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.

### Keywords¶

threshold: [real]

Default: $$10^{-8}$$ (or 1.0d-8).

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.

## solver fft¶

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.

### Keywords¶

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.

## solver scf¶

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.

### Optional 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: $$10^{-6}$$ (or 1.0d-6)

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

gradient threshold: [real]

Default: $$10^{-6}$$ (or 1.0d-6)

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.

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.

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.

## solver scf geoopt¶

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.

### Keywords¶

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

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

restart

Default: false

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

## system¶

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.

### Required keywords¶

name: [string]

Default: none

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

### Optional keywords¶

charge: [integer]

Default: $$0$$

Specifies the charge of the system.

multiplicity: [integer]

Default: $$1$$

Specifies the spin multiplicity of the system.

cartesian gaussians

Default: false

Enables cartesian Gaussians basis functions.

### Examples¶

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


## visualization¶

The visualization section is used for plotting orbitals and densities.

### Keywords¶

grid spacing: [real]

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

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

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.

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

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

plot hf density

Plots the HF density. Optional.

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 cc density

Plots the coupled cluster density. Optional.

Note

This keyword is only read if cc mean value is specified in the do section.