Full solution to the coupled-cluster equations: The H4 model

A version of the homotopy (continuation) methods is successfully applied to calculating the full set of geometrically isolated solutions to a set of nonlinear equations determining the cluster amplitudes in coupled-cluster (CC) methods. The set of equations considered is obtained when applying the CC method, including two-body amplitudes (CCD), to the H4 model. The complete sets of solutions for the spinorbital and spin-symmetry adapted variants of the CCD equations consist of 20 and 12 solutions, respectively. At least 5 solutions are physically meaningful. In addition to standard and nonstandard solutions of the desired symmetry, broken-symmetry solutions are also documented. A preliminary analysis of the significance of the latter solutions is presented. (C) 1998 Elsevier Science B.V. All rights reserved.