24.3 Brueckner coupled-cluster calculations, BCCD

BCCD,[SAVE=record],[PRINT],[TYPE=,type]

BCCD performs a Brueckner coupled-cluster calculation and computes Brueckner orbitals. With these orbitals, the amplitudes of the singles vanish at convergence. Using the BCCD(T) command, the contributions of connected triples are also computed by perturbation theory. Normally, no further input is needed if the BCCD card follows the corresponding HF-SCF. Otherwise, occupancies and orbitals can be specified as in the CI program. BRUECKNER parameters can be modified using the BRUECKNER directive.

The Brueckner orbitals and approximate density matrix can be saved on a MOLPRO dump record using the SAVE option. The orbitals are printed if the PRINT option is given. TYPE can be used to specify the type of the approximate density to be computed:

TYPE=REF

Compute and store density of reference determinant only (default). This corresponds to the BOX (Brueckner orbital expectation value) method of Chem. Phys. Lett. 315, 248 (1999).

TYPE=TOT

Compute and store density with contribution of pair amplitudes (linear terms). Normally, this does not seem to lead to an improvement.

TYPE=ALL

Compute and store both densities

Note: The expectation variables are stored in variables as usual. In the case that both densities are made, the variables contain two values, the first corresponding to REF and the second to TOT (e.g., DMZ(1) and DMZ(2)). If TYPE=REF or TYPE=TOT is give, only the corresponding values are stored.