THE CALCULATION OF MOLECULAR ONE- AND TWO-ELECTRON PROPERTIES FROM MS-XALPHA WAVEFUNCTIONS.

摘要

Part One gives a brief overview of the multiple-scattering X-alpha method. We emphasize the physical interpretation of the X(alpha) model and the effect of the major approximations of the method (statistical exchange and the muffin-tin) on the wavefunction and charge distribution.; Part Two deals with one-electron properties. We use the partitioned-charge method of Case and Karplus: multiplicative one-electron properties are evaluated over the partitioned-charge wavefunction using an operator expansion approach due to Steinborn and Filter. The method is applied to LiH and a variety of planar organic molecules The overall accuracy of calculated properties is intermediate between minimum- and extended-basis Hartree-Fock; moments which weight regions of space close to the nuclei most heavily are the most accurately described. A simple electronegativity picture is developed to interpret the effect of the X(alpha) parameters on the molecular charge distribution.; In Part Three, the methods of Part Two are extended to two-electron properties, specifically electron-repulsion integrals. The coulomb and exchange integrals from our procedure are typically intermediate between MBS and extended-basis HF values. An algorithm is developed to evaluate the Hartree-Fock total energy expression over a self-consistently expanded, partitioned-charge X(alpha) wavefunction: this is used to treat bending and stretching potential surfaces for H(,2)O and O(,3). The two-electron integral procedure is also applied to calculation of the electron-repulsion multiplet structure of excited electronic configurations of O(,3) and VCl(,4).; We examine the question of the form of electron correlation which is contained in (rho)('1/3) density-functional methods by analyzing a number of specific examples. We find that in those cases for which the X(alpha) gives a significant improvement over Hartree-Fock, the underlying reason is the density-functional neglect of artificial differences of ionic character among the single-determinant representations of electronic states.