Determination of atomic states in simple molecules based on perturbation theory in a single-center approach

Abstract

Using diatomic molecules as an example, we study a variational problem concerning the determination of the best atomic wave functions that can be used to construct molecular states. We apply our approach to the one-electron system H2+ for which we formulate a new version of perturbation theory that is correct asymptotically as R → ∞. The new version leads to a good quantitative description of the ground term. Using the Sturm expansion of the Coulomb Green's function we calculate the change of atomic states in first-order perturbation theory and the energies of the 2σg and 2σu terms in the second order. We show that the wave function modifications can only partially be described by introducing effective charges on atoms; corrections that cannot be reduced to a uniform contraction of orbitals are also important.