# Required input sections¶

There are four required input sections for eT:

`system`

`method`

`do`

`geometry`

## 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
```

## 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
```

## 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
```

## 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
```