Externally corrected singles and doubles coupled cluster methods for open-shell systems

The idea of an externally corrected singles and doubles coupled cluster (CCSD) method using an independent source for connected triples and quadruples is generalized to open-shell high-spin states and implemented within the framework of the unitary group based CCSD method. New and more efficient programs are designed to perform cluster analysis of ab initio wave functions and to calculate corrections to standard CCSD equations. The method is applied to describe a single bond breaking of OH in the (2) Pi state, using both double zeta (DZ) and DZ plus polarization (DZP) basis sets. Both full or limited configuration interaction (CI) within small and carefully chosen active spaces (AS) and complete AS self-consistent-held (CASSCF) wave functions are employed as external sources for triple and quadruple corrections. It is shown that the CI or CASSCF corrected CCSD method can effectively account for higher than pair clusters while requiring only a small additional computational effort over that of the standard CCSD approach. (C) 1997 American Institute of Physics.