# UV-Vis Absorption
## Available Coupled Cluster Methods 
- [`CCS`](./../general_sections/jobs.md#CCS)
- [`PE-CCS`](./../embedding_schemes/pe.md#how-to-include-pe)
- [`CC2`](./../general_sections/jobs.md#CC2)
- [`CCSD`](./../general_sections/jobs.md#CCSD)
- [`PE-CCSD`](./../embedding_schemes/pe.md#how-to-include-pe)
- [`CCSDT`](./../general_sections/jobs.md#CCSDT)
## Available Cluster Perturbation Methods
- [`CPS(D)`](./../general_sections/jobs.md#CPS(D))
- [`CPSD(T)`](./../general_sections/jobs.md#CPSD(T))

## Example Input File
An example input file for CCS takes the following form:
```
@method settings
basis: STO-3G
@end

@memory
max_mem: 1
@end

@jobs
task: ccs
@end

@ccs
restart: False
nstates: 3
spectrum: True
@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
| `nstates` <a id="nstates"></a> | |
|----------------|------------------------------------------------------------|
| **Description** | Number of singlet excitation energies to calculate. |
| **Type**        | int |
| **Default**     | 0 |

<br>

| `david_thresh` <a id="david_thresh"></a> | |
|----------------|------------------------------------------------------------|
| **Description** | Convergence threshold for the Davidson solver. |
| **Type**        | float |
| **Default**     | 1.0E-6 |

<br>

| `spectrum` <a id="spectrum"></a> | |
|----------------|------------------------------------------------------------|
| **Description** | Calculation of dipole oscillator strengths. |
| **Type**        | bool |
| **Default**     | False |
| **Availability**| [`CCS`](./../general_sections/jobs.md#CCS)<br>[`PE-CCS`](./../embedding_schemes/pe.md#how-to-include-pe)<br>[`CC2`](./../general_sections/jobs.md#CC2)<br>[`CCSD`](./../general_sections/jobs.md#CCSD)<br>[`PE-CCSD`](./../embedding_schemes/pe.md#how-to-include-pe) |

<br>

| `ntrips` <a id="ntrips"></a> | |
|----------------|------------------------------------------------------------|
| **Description** | Number of triplet excitation energies to calculate. |
| **Type**        | int |
| **Default**     | 0 |
| **Availability**| [`CCS`](./../general_sections/jobs.md#CCS) |

<br>

| `cvs` <a id="cvs"></a> | |
|----------------|------------------------------------------------------------|
| **Description** | Calculate core excitations using core-valence separation (CVS). |
| **Type**        | bool |
| **Default**     | False |
| **Availability**| [`CCS`](./../general_sections/jobs.md#CCS) |

<br>

| `nactivecores` <a id="nactivecores"></a> | |
|----------------|------------------------------------------------------------|
| **Description** | Number of active cores in the CVS calculation. |
| **Type**        | int |
| **Default**     | 1 |
| **Availability**| [`CCS`](./../general_sections/jobs.md#CCS) |

<br>

| `rootwise` <a id="rootwise"></a> | |
|----------------|------------------------------------------------------------|
| **Description** | Perform a pre-optimization of the excitation energies using the Davidson algorithm and converge each individual excitation energy using the DIIS/CROP algorithm. |
| **Type**        | bool |
| **Default**     | False |

<br>

| `preopt_thresh` <a id="preopt_thresh"></a> | |
|----------------|------------------------------------------------------------|
| **Description** | Threshold for the Davidson algorithm used as pre-conditioner for the rootwise solver. |
| **Type**        | float |
| **Default**     | 1.0E-4 |

<br>

| `max_reduced_space` <a id="max_reduced_space"></a> | |
|----------------|------------------------------------------------------------|
| **Description** | Maximal size of the reduced space in the Davidson solver. If this value is exceeded, the construction of the reduced space is restarted from the current eigenvectors. |
| **Type**        | int |
| **Default**     | 8 × [`nstates`](#nstates) / [`ntrips`](#ntrips) |