Distorted-atoms-in molecules model for ordinary covalent molecules. I Description of model; connection with distinguishable electron method; and illustrative application to H2, LiH, and BeH+

Abstract

A simple, but accurate, unrestricted orbital product approximation for the spatial electronic wavefunction of ordinary covalent molecules is developed. Each orbital consists of a pure atomic function which is rescaled in the molecule and, then, polarized by a variable effective internal field. Thus, the molecular orbitals are expressed in terms of known atomic polarization functions. Illustrative calculations on H₂, LiH, and BeH⁺, utilizing single Slater‐type orbitals as the undistorted atomic functions, give 50%–75% of the maximum possible energy improvement over the restricted Hartree‐Fock model. It is shown how our distorted‐atoms‐in‐molecules (DAM) approximation can be employed together with the distinguishable electron perturbation method to determine first‐and second‐order physical properties to within about 10%. We also express the hope that the error in computed electron correlation energies can be reduced to less than 1 kcal/mole per electron pair. Of course, DAM may be used directly to estimate physical properties with medium (∼ 40%) accuracy.