CID
CISD

DESCRIPTION

These method keywords request a Hartree-Fock calculation followed by configuration interaction with all double substitutions (CID) or all single and double substitutions (CISD) from the Hartree-Fock reference determinant [Pople77, Raghavachari80a, Raghavachari81]. CI is a synonym for CISD.

OPTIONS

FC
All frozen core options are available with this keyword. 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 for CISD calculations.

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.

AVAILABILITY

Energies, analytic gradients, and numerical frequencies.

RELATED KEYWORDS

Transformation

EXAMPLES

The CI energy appears in the output as follows:

DE(CI)=    -.48299990D-01        E(CI)=       -.75009023292D+02
NORM(A) =   .10129586D+01

The output following the final CI iteration gives the predicted total energy. The second output line displays the value of Norm(A). Norm(A)–1 gives a measure of the correlation correction to the wavefunction; the coefficient of the HF configuration is thus 1/Norm(A). Note that the wavefunction is stored in intermediate normalization; that is:

Wavefunction in Intermediate Normalization
Wavefunction in Intermediate Normalization

where Ψ0 is the Hartree-Fock determinant and has a coefficient of 1 (which is what intermediate normalization means). Norm(A) is the factor by which to divide the wavefunction as given above to fully normalize it. Thus:

Fully Normalized Wavefunction
Fully Normalized Wavefunction

The coefficient of the Hartree-Fock determinant in the fully normalized wavefunction is then 1/Norm(A), the coefficient of singly-excited determinant Ψi→a is Tia/Norm(A), and so on.


Last updated on: 10 May 2009