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