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+

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.