Application of perturbation theory in large configuration interaction calculations

Rayleigh-Schrödinger perturbation theory has been applied through fifth order in the energy, to the problem of estimating the roots of the secular equation in large configuration interaction calculations. The NO₂⁺, O₃ and H₂O molecules are used as test cases, with accuracy as good as 0.01 eV, with appropriate choice of zero order problem.