# Coupled Cluster

## CCSD

`Fermi.CoupledCluster.RCCSD`

— Type`Fermi.CoupledCluster.RCCSD`

Wave function object for Restricted Coupled Cluster Singles and Doubles.

**High Level Interface**

Run a RCCSD computation and return the RCCSD object:

`julia> @energy rccsd `

Equivalent to

`julia> Fermi.CoupledCluster.RCCSD()`

This function calls a constructor that runs a RCCSD computation based on the options found in `Fermi.Options`

.

**Fields**

Name | Description |
---|---|

`guessenergy` | Energy recovered in the first iteration, normally RMP2 |

`correlation` | Computed RCCSD correlation energy |

`energy` | Total wave function energy (Reference energy + Correlation energy) |

`e_conv` | ΔE from the last iteration |

`t_conv` | Amplitudes RMS change from the last iteration |

**Relevant options**

These options can be set with `@set <option> <value>`

Option | What it does | Type | choices [default] |
---|---|---|---|

`cc_alg` | Picks RCCSD algorithm | `Int` | [1] |

`cc_e_conv` | Energy convergence criterion | `Float64` | [10^-10] |

`cc_max_rms` | Amplitudes RMS convergence criterion | `Float64` | [10^-10] |

`cc_max_iter` | Max number of CC iterations | `Int` | [50] |

`cc_damp_ratio` | Fraction of old amplitudes to be kept | `Float64` | 0.0–1.0 [0.0] |

`cc_diis` | Whether to use DIIS | `Bool` | `false` [`true` ] |

`diis_start` | Iteration number where DIIS starts | `Int` | [3] |

`cc_diis_relax` | Interval between DIIS extrapolations | `Int` | [3] |

`cc_ndiis` | Maximum number of stored vectors for DIIS | `Int` | [3] |

`basis` | What basis set to use | `String` | ["sto-3g"] |

`df` | Whether to use density fitting | `Bool` | `true` [`false` ] |

`rifit` | What aux. basis set to use for RI | `String` | ["auto"] |

`drop_occ` | Number of occupied electrons to be dropped | `Int` | [0] |

`drop_vir` | Number of virtual electrons to be dropped | `Int` | [0] |

## CCSD(T)

`Fermi.CoupledCluster.RCCSDpT`

— Type`Fermi.CoupledCluster.RCCSDpT`

Fermi struct that holds information about RCCSD(T) wavefunctions

*struct tree:*

**RCCSD(T)** <: AbstractCCWavefunction <: AbstractCorrelatedWavefunction <: AbstractWavefunction