Effect of basis set on Mulliken and Löwdin atomic charges, bond orders and valencies of some polar molecules

Abstract

Atomic charges, bond orders and valencies of LiH, LiF, HF, H 2O and NH 3 have been calculated at several geometries using various basis sets and definitions based on Mulliken and Löwdin SCF density matrices. The calculated quantities, although they depend strongly upon basis sets, are found to be rather insensitive to the variation of geometry within a small range. Compared to the corresponding Mulliken values, the Löwdin atomic charges are invariably underestimated, and bond orders and valencies are overestimated. The Mulliken scheme performs more satisfactorily than the Löwdin scheme in the case of predominantly ionic molecules (LiH and LiF). For the polar covalent molecules (HF, H 2O and NH 3), the performance of the two schemes is comparable.