# Linear Response Functions
## Available Methods
Linear response functions are available for:  
- [`CCS`](./../general_sections/jobs.md#CCS)
- [`CC2`](./../general_sections/jobs.md#CC2)
- [`CCSD`](./../general_sections/jobs.md#CCSD)
- [`CC3`](./../general_sections/jobs.md#CC3)
- [`CCSDT`](./../general_sections/jobs.md#CCSDT)

## Example Input File
An example using [`CCS`](./../general_sections/jobs.md#CCS) is given below: 
```
@method settings
basis: STO-3G
@end

@memory
max_mem: 1
@end

@jobs
task: ccs
@end

@ccs
restart: False
solver_thresh: 1.0E-12
operators: DIPLEN ANGMOM
frequency: 0.1
ratio: 1 -1
asym: False
@end

@scf
conv_thresh: 1.0E-12
@end

@molecule
charge: 0
multiplicity: 1
units: bohr
xyz:
O        0.0000000000    0.0000000000    0.0000000000
H        0.0000000000    1.4304281903    1.1071565216
H        0.0000000000   -1.4304281903    1.1071565216
@end
```

## Relevant Keywords

| `operators` <a id="operators"></a> | |
|----------------|------------------------------------------------------------|
| **Description** | Operators used for response functions. <br>- <b>Linear response:</b> Provide two operators<br>- <b>Quadratic response:</b> Provide three operators<br>- <b>Cubic response:</b> Provide four operators |
| **Type**        | str |
| **Default**     | DIPLEN DIPLEN |
| **Options**     | - <b>DIPLEN:</b> Dipole integrals in the length gauge<br>- <b>ANGMOM:</b> Angular momentum integrals<br>- <b>DIPVEL:</b> Dipole integrals in the velocity gauge |


<br>

| `frequency` <a id="frequency"></a> | |
|----------------|------------------------------------------------------------|
| **Description** | Defines which frequencies to calculate response functions for in atomic units. |
| **Type**        | float(s) |
| **Default**     | 0 |
| **Options**     | - If 1 or 2 value(s) are given: Calculate for the listed frequency (frequencies) <br> - If 3 values are given: start stop step |

<br>

| `ratio` <a id="ratio"></a> | |
|----------------------------|--------------------------------------------------------------------------------------------------------------------------------------------------|
| **Description**            | Ratios of frequencies for response functions. <br>- Example for second harmonic generation: 2 -1 -1<br>- Example for third harmonic generation: 3 -1 -1 -1 |
| **Type**                   | floats |
| **Default**                | None |
| **Requirements**           | Must have equivalent length to the [`operators`](#operators) keyword<br>The sum of all ratios must be equal to zero |


<br>

| `asym` <a id="asym"></a> | |
|----------------|------------------------------------------------------------|
| **Description** | Use the asymmetric formulation in which response multipliers are used for linear response functions. |
| **Type**        | bool |
| **Default**     | False |
| **Availability**| **Linear response:**<br>[`CCS`](./../general_sections/jobs.md#CCS)<br>[`CC2`](./../general_sections/jobs.md#CC2)<br>[`CCSD`](./../general_sections/jobs.md#CCSD) |

<br>

| `dipole_pol` <a id="dipole_pol"></a> | |
|----------------|------------------------------------------------------------|
| **Description** | Calculates the components of the polarizability as well as the isotropic and anisotropic polarizability at the given frequency. |
| **Type**        | bool |
| **Default**     | False |