# Target periodic system#

In this section, you can set up the system with spatially periodic boundary condition subject to calculation. This setting is exclusive to the target molecule setting. For periodic systems, the density-fitting approximation is always valid.

## Geometry#

You can specify a molecular structure to be calculated in either Cartesian coordinates or in Z-matrix notation. An error occurs if not specified.

### Atoms#

Enter a list of nuclei that make up the target system.

### Coordinates#

Specify a structure in a cell with a list of coordinates of the nuclei
in the same order as in the list of nuclei `atoms`

. The coordinates of
the molecule should be specified in Å units.

## Dimension#

Specify the number of dimensions with periodicity. The default value is 3.

## Translation Vector#

Enter a list of translation vectors that characterize a periodic
boundary condition with `trans_vector`

key.

## Grid of K-points#

Enter a list of the number of k points for each axis of the reciprocal
space with `kpt_grid_shape`

key.

## Basis set#

You can set up basis functions, which are supported by PySCF. An error occurs if not specified.

## Multiplicity#

Value of the spin multiplicity \(2S+1\) of the target state. An error occurs if not specified or found to be inconsistent with the given number of electrons.

## Number of excited states#

Number of excited states subject to calculation (Default: 0)

## Complete active space#

You can specify a complete active space `cas`

by setting number of
orbitals and number of electrons in the active space.

`active_orb`

: Number of orbitals in the active space.`active_ele`

: Number of electrons in the active space.`cas_list`

: Ordered list of orbtal indices (0-origin) in the active space. If no input is given, the active orbitals will be chosen from around HOMO, LUMO.

You can set different `cas`

for different k with `cas_for_each_k`

key.
It must be given by the list of `cas`

.

## Scaled center#

Shift the k-points to be centered on scaled_center, which is specified as the coefficients of the raciprocal lattice vector. For each element, the value must be a number between -1 and 1.

## Cartesian basis functions#

The basis functions used by PySCF can be specified in Cartesian coordinates for the d and f orbitals (6d, 10f), rather than the irreducible representation of \(S_z\) eigenstates (5d, 7f).

`cart_basis`

: set to true to use Cartesian basis functions (Default: false)

## Effective core potentials#

The effective core potentials passed to PySCF can be specified as a list of dicts.

`ecp`

: a list of dicts, each corresponding to one atom. For each atom set the`atom`

field and the`basis`

fields.`atom`

: atomic species, i.e.`"Na"`

,`"Cu"`

, etc.`basis`

: basis to represent it in, i.e.`"crenbs"`

,`"lanl2dz"`

, etc.

## Atom specific basis#

The basis set used by PySCF can be specified for each atom as a list of dicts (similar to effective core potentials).
This field cannot be set if `basis`

is set.

`atom_specific_basis`

: a list of dicts, each corresponding to one atom. For each atom set the`atom`

field and the`basis`

fields.`atom`

: atomic species, i.e.`"Na"`

,`"Cu"`

, etc.`basis`

: basis to represent it in, i.e.`"sto-3g"`

,`"6311++g**"`

, etc.

## Input example#

```
"target_periodic_system": {
"geometry": {
"atoms": ["H", "H"],
"coordinates": [
[
[0, 0, 0],
[1.42, 0, 0]
]
],
"dimension": 3,
"trans_vector": [
[2.13, -1.2297560733739028, 0],
[2.13, 1.2297560733739028, 0],
[0, 0, 5]
],
"kpt_grid_shape": [2, 1, 1]
},
"basis": "sto-3g",
"num_excited_states": 0,
"cas": {
"active_ele": 2,
"active_orb": 2
},
"multiplicity": 1,
"cart_basis": false
}
```