QCISD Keyword
Last Update: 6/25/2001

This method keyword requests a Quadratic CI calculation [59], including single and double substitutions. Note that this keyword requests only QCISD and does not include the triples correction [267,268,269] by default.

T
Requests a Quadratic CI calculation including single and double substitutions with a triples contribution to the energy added [59].

E4T
Requests a Quadratic CI calculation including single and double substitutions with a triples contribution to the energy and also an evaluation of MP4 triples. Must be specified with the T option.

TQ
Requests a Quadratic CI calculation including single and double substitutions with an energy contribution from triples and quadruples [51] added.

T1Diag
Computes the Q1 diagnostic of T. J. Lee and coworkers [204,270]. Note that Q1 is analogous to the T1 diagnostic for CCSD when it is computed using QCISD instead of the Coupled Cluster method.

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.

CCSD

The predicted energy from a QCISD calculation appears in the output in the final QCISD iteration:

DE(CORR)=  -.54999890D-01       E(CORR)=  -.7501966245dD+02

WhenQCISD(T) is specified, the preceding output is followed by the energy including the non-iterative triples contribution:

QCISD(T)=  -.75019725718D+02

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