# 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
}
```