Variational Ansatz for effective Hamiltonians: solution of convergence problem via the intermediate Hamiltonian approach

Abstract

The variational intermediate Hamiltonian approach is developed on the basis of the quasi-Rayleigh—Schrödinger quasi-degenerate perturbation theory. The results of test applications to ab initio calculations of molecular excited states are discussed. The new approach is able to produce rapidly and monotonically convergent sequences of energy estimates while the ordinary variational effective Hamiltonian method suffers from catastrophic divergencies in the presence of intruder states.