28 THE CLOSED SHELL CCSD PROGRAM

Bibliography:

C. Hampel, K. Peterson, and H.-J. Werner, Chem. Phys. Lett. 190, 1 (1992)

All publications resulting from use of this program must acknowledge the above.

The CCSD program is called by the CISD, CCSD, BCCD, or QCI directives. CID or CCD can be done as special cases using the NOSINGL directive. The code also allows to calculate Brueckner orbitals (QCI and CCSD are identical in this case). Normally, no further input is needed if the CCSD card follows the corresponding HF-SCF. Optional ORBITAL, OCC, CLOSED, CORE, SAVE, START, PRINT options work as described for the MRCI program in section 21. The only special input directives for this code are BRUECKNER and DIIS, as described below.

The following options may be specified on the command line:

NOCHECK

Ignore convergence checks.

DIRECT

Do calculation integral direct.

NOSING

Do not include singly external configurations.

MAXIT=value

Maximum number of iterations.

SHIFTS=value

Denominator shift for update of singles.

SHIFTP=value

Denominator shift for update of doubles.

THRDEN=value

Convergence threshold for the energy.

THRVAR=value

Convergence threshold for CC amplitudes. This applies to the square sum of the changes of the amplitudes.

The convergence thresholds can also be modified using

THRESH,ENERGY=thrden,COEFF=thrvar

Convergence is reached if the energy change is smaller than thrden (default 1.d-6) and the square sum of the amplitude changes is smaller than thrvar (default (1.d-10). The THRESH card must follow the command for the method (e.g., CCSD) and then overwrites the corresponding global options (see GTHRESH, sec. 6.11).

The computed energies are stored in variables as explained in section 8.8. As well as the energy, the $T_1$ diagnostic (T. J. Lee and P. R. Taylor, Int. J. Quant. Chem. S23 (1989) 199) is printed and stored in the variable T1DIAG for later analysis.