The choice of orbitals and single particle energies for the evaluation of molecular energies by a variationhyphen;perturbation approach

摘要

A multireference perturbativehyphen;variational method, proposed by the authors to compute the molecular energies and wave functions is investigated to assess the importance of the choice of the orbital basis and of the lsquo;lsquo;zeroth orderrsquo;rsquo; Hamiltonian on the computed energies. A new definition is presented of single particle energy suitable to perform perturbativehyphen;variational computations with any type of orbital sets. Test computations performed with various orbital sets for the Be+H2system at various geometries indicate that the computed energies are not very sensitive to the choice of the single particle energies but they are sensitive to the choice of the orbitals. The MCSCF orbitals appear to provide the best starting set of orbitals to perform this type of computation. The computed energies are compared to those evaluated by single and multireference coupled cluster, MBPT, and CI methods.