MOLPRO

24 THE CLOSED SHELL CCSD PROGRAM 183
24.2 Quadratic configuration interaction, QCI
QCI or QCISD performs quadratic configuration interaction, QCISD. Using the QCI(T) or
QCISD(T) commands, the contributions of connected triples are also computed by perturbation
theory. Normally, no further input is needed if the QCI card follows the corresponding HF-SCF.
Otherwise, occupancies and orbitals can be specified as in the CI program. For modifying DIIS
directives, see section 24.5. For first-order QCISD and QCIDF(T) properties see section .
For avoiding error exits in case of no convergence, see CCSD(T).
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
TYPE=TOT
TYPE=ALL
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).
Compute and store density with contribution of pair amplitudes (linear
terms). Normally, this does not seem to lead to an improvement.
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).
24.3.1 The BRUECKNER directive
BRUECKNER,orbbrk,ibrstr,ibrueck,brsfak;
This directive allows the modification of options for Brueckner calculations. Normally, none of
the options has to be specified, and the BCCD command can be used to perform a Brueckner
CCD calculation.
orbbrk:
ibrstr:
ibrueck:
brsfak:
if nonzero, the Brueckner orbitals are saved on this record.
First iteration in which orbitals are modified (default=3).
Iteration increment between orbital updates (default=1).
Scaling factor for singles in orbital updates (default=1).