Hilbert space multireference coupled-cluster methods. I. The single and double excitation model

Employing a Hilbert space ansatz the multireference, coupled-cluster single and double (MRCCSD) excitation equations are derived and presented in a transparent diagrammatic form for a complete active multireference space. All possible excitation levels are considered to be in the complete active space through hextuples, for which the MRCCSD equations are shown to be complete. The equations apply for open- and closed-shell cases.