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# CID and CISD

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.

#### 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 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.

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:

$\Psi^{CISD} = \Psi^0 + \sum\limits_{ia}T_{ia}\Psi(i{\rightarrow}a)+\sum\limits_{iajb}T_{ia}\Psi(ij{\rightarrow}ab)$
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:

$\mathrm{Norm(A)}=\sqrt{(1+\sum\limits_{ia}T_{ia}T_{ia}+\sum\limits_{ijab}T_{ijab}T_{ijab})}$
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.