CID and CISD Keywords
Last Update: 6/26/2001

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 [48,95,117]. CI is a synonym for CISD.

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 for CISD calculations.

Analytic energies, analytic gradients, and numerical frequencies.

Transformation

The CI energy appears in the output as follows:

DE(CI)=    -.54192458D-01        E(CI)=       -.75018855026D+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:

where Y⁰ 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:

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

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