# Coupled Cluster Theory

Coupled Cluster (CC) theory is a cornerstone of quantum chemistry, providing highly accurate solutions to the electronic Schrödinger equation for many-body systems. These methods offer a range of accuracy and computational cost, allowing users to choose based on their specific needs and resources.

## Available Coupled Cluster Tasks

| Task                                              | Description                                                     |
|---------------------------------------------------|-----------------------------------------------------------------|
| [`CCS`](./../general_sections/jobs.md#CCS)         | Coupled Cluster Singles.                                        |
| [`CC2`](./../general_sections/jobs.md#CC2)         | Approximate Coupled Cluster Singles and Doubles.                |
| [`CCSD`](./../general_sections/jobs.md#CCSD)       | Coupled Cluster Singles and Doubles.                            |
| [`RR-CCSD`](./../general_sections/jobs.md#RR-CCSD) | Rank-Reduced CCSD, a computationally efficient variant of CCSD. |
| [`CC3`](./../general_sections/jobs.md#CC3)         | Coupled Cluster Singles, Doubles, and approximate Triples.      |
| [`CCSD(T)`](./../general_sections/jobs.md#CCSD(T)) | CCSD with non-iterative, perturbative treatment of Triples.     |
| [`CCSDT`](./../general_sections/jobs.md#CCSDT)     | Coupled Cluster Singles, Doubles, and Triples.                  |
| [`CCSDTQ`](./../general_sections/jobs.md#CCSDTQ)   | Coupled Cluster Singles, Doubles, Triples, and Quadruples.      |

## General Keywords for CC Tasks

| `solver_thresh` <a id="solver_thresh"></a> | |
|----------------|-------------------------------|
| **Description** | Convergence threshold for the CC solver. |
| **Type**        | float |
| **Default**     | 1.0E-06 |

<br>

| `max_iter` <a id="max_iter"></a> | |
|----------------|------------------------------------------|
| **Description** | Maximum number of iterations allowed in the CC solver. |
| **Type**        | int |
| **Default**     | 100 |

<br>

| `crop` <a id="crop"></a> | |
|----------------|----------------------------------------------------------|
| **Description** | Whether CROP acceleration should be used in the CC solver. If False, DIIS is used. |
| **Type**        | bool |
| **Default**     | True |

<br>

| `diis_max` <a id="diis_max"></a> | |
|----------------|----------------------------------------------|
| **Description** | Number of CROP/DIIS vectors stored at a time in the CC solver. |
| **Type**        | int |
| **Default**     | 10 |

<br>

| `restart` <a id="restart"></a> | |
|----------------|--------------------------------------------------|
| **Description** | Enables restarting the calculation from a checkpoint file in a .h5 format. |
| **Type**        | bool |
| **Default**     | True |

<br>

| `frozencore` <a id="frozencore"></a> | |
|----------------|--------------------------------------------------|
| **Description** | Enables freezing of core orbitals to reduce computational cost. |
| **Type**        | bool |
| **Default**     | False |

<br>

| `cholesky` <a id="cholesky"></a> | |
|----------------|----------------------------------------------------------------------------------|
| **Description** | Enables Cholesky decomposition of AO electron repulsion integrals (ERIs). |
| **Type**        | bool |
| **Default**     | False |
| **Availability** |[`CCS`](./../general_sections/jobs.md#CCS)<br>[`CC2`](./../general_sections/jobs.md#CC2)<br>[`CCSD`](./../general_sections/jobs.md#CCSD) |

<br>

| `max_cholesky` <a id="max_cholesky"></a> | |
|----------------|---------------------------------------------------------------------|
| **Description** | Upper limit of the number of Cholesky vectors allowed. In practical cases cholesky thresh defines the used amount of Cholesky vectors. |
| **Type**        | float |
| **Default**     | 1.0E-09 |

<br>

| `cholesky_thresh` <a id="cholesky_thresh"></a> | |
|----------------|-----------------------------------------------------------|
| **Description** | Threshold for constructing Cholesky vectors. |
| **Type**        | float |
| **Default**     | [`solver_thresh`](#solver_thresh) / 1.0E-02 |

<br>

| `reduce_print` <a id="reduce_print"></a> | |
|----------------|----------------------------------------------|
| **Description** | Suppresses iteration information from the linear solver. |
| **Type**        | bool |
| **Default**     | True |

<br>

| `rr-cutoff` <a id="rr-cutoff"></a> | |
|----------------|----------------------------------------------------------|
| **Description** | Threshold for eigenvalues in the projector for Rank-Reduced CCSD. |
| **Type**        | float |
| **Default**     | 1.0E-4 |
| **Availability** | [`RR-CCSD`](./../general_sections/jobs.md#RR-CCSD) |

<br>

| `projector_type` <a id="projector_type"></a> | |
|----------------|----------------------------------------------------|
| **Description** | Type of projector used in Rank-Reduced calculations. |
| **Type**        | str |
| **Default**     | MP2 |
| **Availability** | [`RR-CCSD`](./../general_sections/jobs.md#RR-CCSD) |
| **Options**     | MP2<br>MP3|