Note

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C.A.S.

Adjust the active orbitals that define a Complete Active Space (CAS) in an INDO calculation.

SYNTAX: C.A.S.=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.A.S.=(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.

A CAS contains all spin-adapted configurations of electrons rearranged within the active orbitals.

By default, 2 orbitals are actived when MRCI is specified (equivalent to C.A.S.=2).

If the C.A.S. keyword is combined with other keywords related to active spaces (MRCI, CIS, CISD, C.I., C.I.D.), the CAS is further expanded by applying single-electron or double-electron excitations to its configurations.

Note that the size of CAS grows exponentially with \(n\). Only a small number of orbitals can be activated before the computational costs becomes prohibitive. This is particularly true if the active space is further expanded by additional few-electron excitations.