Coupled-cluster theory and the method of moments

Following an idea by Jankowski et al. (Theor. Chim. Acta 80, (1991), 223), we investigate the effect of minimizing the sum of squared moments of the Hamiltonian with respect to the coupled-cluster singles and doubles (CCSD) amplitudes. If considering moments built with just singly and doubly excited functions, the ordinary CCSD amplitudes are recovered. When including higher excitations, the resulting energies are usually worse than those obtained from standard CCSD theory, but moment-optimized CCSD wave functions are slightly more accurate than standard ones. Examples where moment-minimized CCSD energies are more realistic are represented by the dissociation profile of the N2 molecule described by single reference CCSD, or the potential curve of the water molecule for symmetric dissociation.