EXTERNALLY CORRECTED SINGLES AND DOUBLES COUPLED-CLUSTER METHODS FOR OPEN-SHELL SYSTEMS - II - APPLICATIONS TO THE LOW-LYING DOUBLET STATESOF OH, NH2, CH3 AND CN RADICALS

The externally-corrected singles and doubles coupled cluster (CCSD) me thod, as implemented for high-spin open shell systems by exploiting th e unitary group approach and restricted to the first order interacting space (UGA-CCSD(is)) in Part I (Li, X., et ai., 1997, J. chem. Phys., 107, 90), is applied to several simple radicals in their doublet grou nd and excited states. The capabilities and limitations of this approa ch are examined by studying the potential energy surfaces or their sui table cuts involving the dissociation of both single and multiple bond s (OH and CN) or simultaneous dissociation of several single bonds (NH 2 and CH3). Using low dimensional CAS-FCI and SOCI wave functions for the internal and external active space excitations, it is shown that c orrected 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 cluste rs even in severe cases of quasi-degeneracy.