On the energies of low-lying excited electronic states of molecules

Abstract

Two methods for calculating the energies of low-lying excited electronic states of molecules of the same symmetry as the ground state are compared. We term these the constrained variational method and the optimized trace method . Approximate solutions of the Schrodinger equation can be obtained by introducing finite basis set expansions. For each method two possibilities arise: (i) a common basis set can be used for the ground and the excited state; (ii) different basis sets can be employed for different states. Prototype calculations are reported for the hydrogen molecular ion using distributed basis sets of s-type Gaussian functions.