Representations of molecular force fields. V. On the equilibrium structure of methane

Abstract

The correction from mean (rg) to equilibrium (re) bond lengths in methane is reinvestigated in response to suggestions that the original Kuchitsu–Bartell (KB) corrections were excessive by as much as 0.006 Å. It is found that the KB model anharmonic force field with its original parameterization does overcorrect, relative to the new Pulay ab initio force field, but only by 10% of the original (rg−re) corrections, or 0.0022 Å for CH4 and 0.0015 Å for CD4. The KB model with the more recent MUB-2 nonbonded parameterization gives results in close accord with the ab initio results, supporting the utility of the augmented anharmonic Urey–Bradley representation of force fields. Reported discrepancies between experimental and theoretical methane bond lengths cannot be due solely to the form of or original parameterization of the KB model field.