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C.I.D.

Adjusts the active orbitals for double-electron excitations in an INDO calculation with the CISD keyword specified.

SYNTAX: C.I.D.=n

Activate the \(\lceil n/2 \rceil\) orbitals at or below the chemical potential and the \(\lfloor n/2 \rfloor\) orbitals above the chemical potential.

SYNTAX: C.I.D.=(n,m)

Activate the \(n\) orbitals closest to the chemical potential containing \(m\) electron pairs. This accounting ignores the unpaired electron in systems with an odd number of electrons.

By default, 10 orbitals are activated when CISD is specified (equivalent to C.I.D.=10).

Doubly-excited configurations are generated by adding single-electron excitations to the singly-excited configurations in the active space. This process requires the active orbitals for double excitations to be a subset of the active orbitals for single excitations, and the values supplied by C.I.D. are automatically adjusted to satisfy this constraint.

For a CISD calculation with the active orbitals set by C.I.=(p,o) and C.I.D.=(n,m), the number of doubly-excited configurations is approximately \(m(m-n)o(p-o)\).

Even if the MAXCI keyword is used to limit the size of the CI Hamiltonian matrix, generating and computing the energies of doubly-excited electron configurations can become computationally expensive if a large number of orbitals are activated by this keyword.