Representation of electron densities. 1. Sphere fits to total electron density surfaces

A method is detailed enabling fits to calculated total electron densities using spheres of adjustable radii centeredon the nuclear positions. Best atomic radii are obtained for a series of molecules with use of Hartree-Fock models. Minimalsplit-valence and polarization basis set results are compared. The dependency of atom size (in molecules) on the electronegativityof attached groups and on steric and strain conditions is examined, and a set of average radii for use in molecular modelsis proposed. Comparisons between calculated radii and atomic charges obtained from Mulliken population analyses suggestthat the latter method generally underestimates the population of hydrogen and overestimates the population of lithium andsodium