CCD and CCSD Keywords
Last Update: 6/26/2001

These method keywords request a coupled cluster [54,203] calculations, using double substitutions from the Hartree-Fock determinant for CCD [54], or both single and double substitutions for CCSD [55,56,57,58]. CC and QCID are synonyms for CCD.

T
Include triple excitations non-iteratively [59] (CCSD only). CCSD-T is a synonym for CCSD(T).

E4T
Used with the T option to request inclusion of triple excitations for both the complete MP4 and to form CCSD(T).

T1Diag
Computes the T1 diagnostic of T. J. Lee and coworkers [204] (CCSD only).

FC
This indicates "frozen-core," and it implies that inner-shells are excluded from the correlation calculation. This is the default calculation mode. Full specifies the inclusion of all electrons, and RW and Window allow you to input specific information about which orbitals to retain in the post-SCF calculation (see the discussion of the MPn keywords for an example).

Conver=N
Sets the convergence calculations to 10^-N on the energy and 10^-(N+2) on the wavefunction. The default is N=7 for single points and N=8 for gradients.

MaxCyc=n
Specifies the maximum number of cycles for CCSD calculations.

Analytic energies, analytic gradients for CCD and numerical gradients for the other methods, and numerical frequencies.

MP4, QCISD, Transformation

The Coupled Cluster energy appears in the output as follows (following the final correlation iteration):

DE(CORR)=  -.54979226D-01        E(CORR)=     -.75019641794D+02
 ...
CCSD(T)=  -.75019717665D+02

The CCSD energy is labeled E(CORR), and the energy including the non-iterative triples contribution is given in the final line.

Return to TOC