The coupled cluster approach is one of the most accurate and reliable quantum
chemical techniques for including the effects of electron correlation.
PSI3 is capable of computing energies, analytic gradients, and
linear response properties using a number of coupled cluster models.
Table 8 summarizes these capabilities. This section
describes how to carry out coupled cluster calculations within PSI3.
| Reference | Method | Energy | Gradient | Exc. Energies | LR Props |
| RHF | CC2 | Y | N | Y | Y |
| UHF | CC2 | Y | N | Y | N |
| ROHF | CC2 | Y | N | Y | N |
| RHF | CCSD | Y | Y | Y | Y |
| RHF | CCSD(T) | Y | N | - | - |
| ROHF | CCSD | Y | Y | Y | N |
| ROHF | CCSD(T) | N | N | - | - |
| UHF | CCSD | Y | Y | Y | N |
| UHF | CCSD(T) | Y | Y |
- | - |
| Brueckner | CCD | Y | N | N | N |
| Brueckner | CCD(T) | Y | N | - | - |
CCSD(T) gradients implemented via an experimental code.
A more efficient and robust implementation will appear in the next
release.