A general model-space coupled-cluster method using a Hilbert-space approach

Abstract

An extension of the recently reported multireference coupled-cluster method [L. Meissner, S. A. Kucharski, and R. J. Bartlett, J. Chem. Phys. 91, 6187 (1989)] to the general incomplete model space is presented. Connectedness of the cluster operators and the effective Hamiltonian is achieved by the admission of a special kind of internal excitation in the standard definition of the cluster operators and by setting corresponding matrix elements of the effective Hamiltonian equal to zero. This special type of similarity transformation leads to a unique partition of the excitations from the model space into those generated by the cluster operators and those generated by the effective Hamiltonian. The new form of the effective Hamiltonian preserves the size extensivity of the method. A discussion of the application of the method to the problem of the simultaneous calculation of energies of the ground closed-shell state and excited states is considered.