Complete set of solutions of multireference coupled-cluster equations: Thestate-universal formalism - art. no. 052506

The homotopy method is used to obtain a complete set of solutions of multir eference coupled-cluster equations within the framework of the state-univer sal coupled-cluster (SUCC) formalism. The solutions are obtained for an ab initio four-electron model system consisting of four hydrogen atoms in rect angular arrangement. Essential information about the structure of solutions of nonlinear SUCC equations is provided and analyzed, both numerically and analytically, in the form of two theorems. The existence of multiple solut ions of SUCC equations provides new insights into the nature of convergence problems often encountered in SUCC calculations. The evidence for the poss ible association of some solutions of the SUCC equations with the intruder- state problem is presented and briefly discussed.