C.A.S=n, C.A.S.=(n,m)

Available for INDO-based CI calculations only. A complete active space calculation will be performed within an active space of n M.O.s. If only n is specified, the n M.O.s which ‘bracket’ the occupied-virtual energy levels are selected. (see C.I.=n for details). In C.A.S.=(n,m), n is the number of M.O.s in the active space, and m is the number of doubly filled (that is, not empty or partially filled) levels to be used. If an MRCI calculation is requested without specifying C.A.S., the default active space for the C.A.S. reference determinants is (2,1).

If the C.A.S. keyword is combined with another CI-related keyword (MRCI, CIS, CISD, C.I., or C.I.D.), the excitations generated by C.A.S. will be used as reference determinants to generate single excitations (or single and double excitations, for CISD and C.I.D.).

Note that the number of excitations in C.A.S. scales exponentially with size of the active space; only a small handful of molecular orbitals can be included in the C.A.S. active space before the computational cost becomes prohibitive. This is particularly true if the C.A.S. determinants are used as reference determinants for MRCI.