Comparison of configuration interaction expansions based on different orbital transformations

Abstract

The characteristics of CI expansions based on different types of orbitals (all obtained from the same basis set) are compared in a series of calculations on the water molecule. The orbitals compared include canonical SCF molecular orbitals, natural orbitals, and two other types related to these. The comparisons include energy and property convergence and the distribution of magnitudes of the off-diagonal elements of the Hamiltonian matrix and of the eigenvector components. The implications of the results are discussed.