IMPROVED COMPUTATIONAL STRATEGY FOR THE STATE-SELECTIVE COUPLED-CLUSTER THEORY WITH SEMIINTERNAL TRIEXCITED CLUSTERS - POTENTIAL-ENERGY SURFACE OF THE HF MOLECULE

The recently developed state-selective (SS) multi-reference coupled-cl uster (CC) method involving all singly and doubly, and semi-internal t riply excited clusters from the formal reference configuration [SSCCSD (T) approach] is tested in the calculation of the potential energy sur face (PES) of the HF molecule. Both double zeta and double zeta plus p olarization basis sets are employed and a few different choices of act ive space are considered. The SSCCSD(T) method provides an accurate de scription of the entire RES at low cost even for the bond breaking reg ion, contrary to the results obtained with the perturbative single-ref erence CCSD(T) method or various limited configuration interaction app roaches. This is the first application of the new SSCC code, which use s an improved computational strategy for handling the semi-internal tr iexcited clusters. Details of this new implementation of the SSCCSD(T) method are discussed. (C) 1995 American Institute of Physics.