Available for INDO-based CI calculations only. Use multi-reference configuration interaction. The references for MRCI can be selected using the C.A.S. keyword; by default, the active space is two M.O.s. If CIS or CISD is not specified, CIS is assumed.
Multi-reference CI is useful for generating specific double and higher excitations. Within multi-reference CI, the reference determinants are used as starting points for generating single excitations (or single and double excitations, if CISD is specified). For a closed-shell system using the default active space, the reference determinants are:
1. The SCF ground state
1. A single (HOMO → LUMO) excitation
2. A double (HOMO, HOMO → LUMO, LUMO) excitation
The single excitation (HOMO x → LUMO + y) starting from reference determinant 1 will yield a single excitation, as in CIS. The same excitation starting from reference determinant 2 will yield the double excitation (HOMO x, HOMO → LUMO, LUMO + y). Similarly, the same excitation starting from reference determinant 3 will yield the triple excitation (HOMO x, HOMO, HOMO → LUMO, LUMO, LUMO + y).