This method keyword requests a Quadratic CI calculation [Pople87], including single and double substitutions. Note that this keyword requests only QCISD and does not include the triples correction [Gauss88, Salter89] by default (see T in Options).
T
Requests a Quadratic CI calculation including single and double substitutions with a triples contribution to the energy added [Pople87].
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 [Raghavachari90] added.
SaveAmplitudes
Saves the converged amplitudes in the checkpoint file for use in a subsequent calculation (e.g., using a larger basis set). Using this option results in a very large checkpoint file, but also may significantly speed up later calculations.
ReadAmplitudes
Reads the converged amplitudes from the checkpoint file (if present). Note that the new calculation can use a different basis set, method (if applicable), etc. than the original one.
T1Diag
Computes the Q1 diagnostic of T. J. Lee and coworkers [Lee89, Lee90]. Note that Q1 is analogous to the T1 diagnostic for CCSD when it is computed using QCISD instead of the Coupled Cluster method.
FC
All frozen core options are available with this keyword; a frozen core calculation is the default. See the discussion of the FC options for full information.
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. The default is 50.
Analytic energies and gradients for QCISD, numerical gradients for QCISD(T), and numerical frequencies for all methods.
The predicted energy from a QCISD calculation appears in the output in the final QCISD iteration:
DE(CORR)= -.54999890D-01 E(CORR)= -.7501966245D+02
When QCISD(T) is specified, the preceding output is followed by the energy including the non-iterative triples contribution:
QCISD(T)= -.75019725718D+02
