Extended Hartree—Fock Wavefunctions: Optimized Valence Configurations for H2 and Li2, Optimized Double Configurations for F2

Abstract

As a step beyond the Hartree—Fock technique in the search for better energies, wavefunctions, and the general description of molecular formation and dissociation, a configuration-mixing method of the following nature is developed and illustrated for H2, Li2, and F2. The wavefunction Ψ= ∑ kAkΦkconsists of several optimized configurations obtained by replacing one of the σ-orbitals of the primary configuration (in our case the Hartree—Fock) by orbitals of different kinds. All the orbitals involved are orthonormalized, insuring orthonormalization of the configurations themselves. The present method determines energetically the optimum combination of both the mixing coefficients Ak and the linear orbital parameters by the solution of SCF-type equations and insures dissociation of the molecule to two Hartree—Fock atoms. A program based upon this formalism has been constructed by the authors for the IBM 7094 computer and is capable of handling homonuclear diatomic molecules using as many as ten configurations with up to 33 two-center symmetry basis functions. Results obtained are energetically better than conventional multiconfiguration studies and since our method is designed to account for added correlation energy associated with molecular formation, calculated potential curves much more realistic than the Hartree—Fock ones can be realized. Sample calculations which represent a partial-optimization procedure within the framework of the analysis are given for the molecules H2, Li2, and F2. We feel, however, that these results are quite close to the true solution. The binding energies obtained with these optimized configuration functions are 4.63, 0.93, and 0.54 eV for H2, Li2, and F2, respectively, as contrasted with the Hartree—Fock dissociation energies of 3.64, 0.17, and −1.37 eV.