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.
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.
Specifies the maximum number of cycles for CISD calculations.
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.
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.
Energies, analytic gradients, and numerical frequencies.
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
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
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 October 2016. [G16 Rev. A.03]