MULTIPLE SOLUTIONS OF THE SINGLE-REFERENCE COUPLED-CLUSTER EQUATIONS .2. ALTERNATIVE REFERENCE STATES

Single-reference coupled-cluster (sn-cc) methods parametrized with respect to four alternative reference-state configurations and orbital sets are applied to the simple H4 model system in which the degree of quasi-degeneracy of the electronic states can be varied in a wide range. Both the ground state and 10 excited states are considered with the aim of numerically studying the attainability and properties of multiple solutions of the system of nonlinear equations for the cluster amplitudes. Comparisons of up to four alternative descriptions of a given state in terms of various solutions of the sn-ce equations are made. It is found that for some excited states different parametrizations yield very close results and that the classification of the solutions into standard and nonstandard ones may not be straightforward. (C) 1995 John Wiley & Sons, Inc.