MULTIPLE SOLUTIONS OF THE VALENCE-UNIVERSAL COUPLED-CLUSTER EQUATIONS FOR BE, B+, AND C-2+

Sets of nonlinear equations for the cluster amplitudes of the valence-universal coupled-cluster (VU-CC) method have been solved to obtain physically meaningful multiple solutions for Be, B+, and C2+. The wave operator is taken in Lindgren's normal ordered exponential form and the completeness of the model space is postulated. The cluster operator is restricted to its one- and two-electron components that are represented in terms of radial amplitudes defined by the configurational excitations (VU-CCSD/R method). Five solutions giving rise to 10 approximate energies of four 1S states are obtained and discussed, These are the first multiple solutions documented for a nonmodel system. Some attention is paid to the problem of the efficiency of various methods in obtaining alternative solutions and to some consequences of the availability of alternative solutions. (C) 1993 John Wiley & Sons, Inc.