Externally corrected singles and doubles coupled cluster methods for open-shell systems. II. Applications to the low lying doublet states of OH, NH2, CH3 and CN radicals

Abstract

The externally-corrected singles and doubles coupled cluster (CCSD) method, as implemented for high-spin open shell systems by exploiting the unitary group approach and restricted to the first order interacting space (UGA-CCSD(is)) in Part I (Li, X., et al., 1997, J. chem. Phys., 107,90), is applied to several simple radicals in their doublet ground and excited states. The capabilities and limitations of this approach are examined by studying the potential energy surfaces or their suitable cuts involving the dissociation of both single and multiple bonds (OH and CN) or simultaneous dissociation of several single bonds (NH2 and CH3). Using low dimensional CAS-FCI and SOCI wave functions for the internal and external active space excitations, it is shown that corrected CCSD energies are superior to the standard ones in all cases, including those obtained with CI spaces of very modest dimension, and are capable of accounting for the presence of higher than pair clusters even in severe cases of quasi-degeneracy.