24.3 Brueckner coupled-cluster calculations, BCCD

`BCCD`,[`SAVEORB=`*record*],[`PRINT`],[`TYPE=`,*type*]

In addition, the same options as for CCSD can be used.

`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 `SAVEORB` 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.

For avoiding error exits in case of no convergence, see `CCSD(T)`.