Relationship between configuration interaction and coupled cluster approaches

A variational principle for the linear coupled pair many-electron theory (L-CPMET) is given and its implications for the relationship between certain configuration interaction (CI) and coupled cluster approaches (CCA), and for the computational aspects of solving the algebraic systems occurring in CCA, are discussed. An exact relationship between the correlated energies and wave functions as obtained with the L-CPMET and with the CI limited to at most doubly excited configurations (D-CI) is derived and used to provide a new viewpoint on the origins of Davidson’s correction for unlinked cluster contributions in the D-CI energy. The results are illustrated on the CO molecule.