Eigenvalue problem formulation of coupled-cluster expansions through intermediate Hamiltonians

Using the intermediate Hamiltonian formalism, it is shown that coupled-cluster doubles (CCD) or singles and doubles (CCSD) may be formulated as an eigenvalue problem by dressing properly the doubles or singles and doubles configuration interaction matrix. The dressing may be either diagonal or non-diagonal. The so-formulated CC method converges even in nearly degenerate situations where the traditional set of coupled equations faces usually divergent behaviour.