Diagrammatic non-degenerate RSPT for evaluation of ground-state energy of simple open-shell molecular systems

The diagrammatic non-degenerate Rayleigh-Schrödinger perturbation theory and the coupled cluster approach are applied to the evaluation of the ground state energy of a simple open-shell molecular system described in the zeroth order approximation by a single determinant wavefunction. The corresponding hamiltonian is built up by the creation and annihilation operators introduced over an orthonormal set of restricted Hartree-Fock one-particle functions. The perturbation H₁ is composed of one- and two-particle terms, the one-particle term depending in an explicit way upon the type of restricted Hartree-Fock theory used. The efficiency of the elaborated theory is illustrated for the BeH molecule.