Direct iterative solution of the generalized Bloch equation. II. A general formalism for many-electron systems

Abstract

A general nonperturbative formulation of the recently proposed [H. Meißner and E. O. Steinborn, Int. J. Quantum Chem. 61, 777 (1997); Part I] quadratic iterative scheme for the wave function expansion coefficients (WECs), enabling a direct solution of the generalized Bloch equation, is given for the ab initio electronic Hamiltonians, thus enabling the computation of the molecular electronic structure. The method exploits the concepts of a multidimensional reference or model space, a (non-Hermitian) effective Hamiltonian, and the generalized Bloch equation. The formulation in terms of WECs provides a considerable freedom in the design of various approximation schemes by combining direct iterations on WECs with their approximation by disconnected cluster components based on the exponential cluster ansatz for the wave operator. The resulting formalism is capable of handling a rather large class of both ground and excited states. While the general formulation represents a multiconfigurational, multireference scheme, a special attention is paid to its two-dimensional state selective or state specific version.