Constrained molecular wavefunctions: HF molecule

Abstract

A simple approach for supplementing the usual variational energy criterion for molecular wavefunctions is described. With the requirement that integrals of selected operators agree with the known experimental or theoretical values for the corresponding properties, a constrained variation method is formulated. For an appropriate choice of one-electron operators, the resulting charge distribution can be of greater accuracy than that obtained from an unrestricted variation treatment with the same basis set. Since the method is of most interest for simple wavefunctions, it is illustrated in terms of a modification of the best-limited LCAOMO hydrogen fluoride function of Ransil. A comparison of the two functions shows that the constrained variation treatment leads to improved results for certain one-electron properties. The calculations also demonstrate the sensitivity of the one-electron expectation values to small changes in the orbital coefficients and the total energy.