In CCSD the ground-state energy is (as for CC2) evaluated as

E_{CC} |
= 〈HF| H| CC〉 = 〈HF| H exp(T)| HF〉 , |
(10.1) |

where the cluster operator

T_{1} |
= t_{ai}τ_{ai} , |
(10.2) |

T_{2} |
= t_{aibj}τ_{aibj} . |
(10.3) |

In difference to CC2, the cluster amplitudes

Ω_{μ1} |
= 〈μ_{1}| + [, T_{2}]| HF〉 = 0 , |
(10.4) |

Ω_{μ2} |
= 〈μ_{2}| + [, T_{2}] + [[, T_{2}], T_{2}]| HF〉 = 0 , |
(10.5) |

where again

= exp(- *T*_{1})exp(*T*_{1}),

and

- Explicitly-correlated CCSD(F12) methods:
- CC calculations with restricted open-shell (ROHF) references:
- Perturbative triples corrections:
- Integral-direct implementation and resolution-of-the-identity approximation:
- Disc space requirements:
- Memory requirements:
- Important options:
- Excitation energies with CCSD: