Orthogonality constrained general variational calculation of excited state wavefunctions and energies: a new strategy

Abstract

A viable strategy is developed for the general variational calculation of excited state wavefunctions which are constrained to remainorthogonal to all the lower-lying states of the samesymmetry. A key element of the strategy is to employ the penalised functional procedure for enforcing the relevant orthogonality constraints and the method of steepest descent to locate the constrained minima with respect to all variables, linear as well as nonlinear. The workability of the algorithm is tested by applying the technique for the optimization of nonlinear parameters in trial functions for the 2_s state of H atom and singlet 1_s2_s states of helium atom and some isoelectronic ions.