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

Two methods for calculating the energies of low-lying excited electronic st ates of molecules of the same symmetry as the ground state are compared. We term these the constrained variational method and the optimized trace meth od. Approximate solutions of the Schrodinger equation can be obtained by in troducing finite basis set expansions. For each method two possibilities ar ise: (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 di stributed basis sets of s-type Gaussian functions.