Valence States in Molecules. 3. Transferable Vibrational Force Constants from Homonuclear Data

A recently introduced valence-state potential energy function (von Szentpály, L. Chem. Phys. Lett. 1 995, 245, 209.) and its dissociation energy ( D VS ) serve to establish an arithmetic combining rule for scaled force constant increments, λ = kR e / D VS . λ is transferable from homo- to heteronuclear molecules at an accuracy of 1.6% average absolute error in harmonic vibrational wavenumbers, ṽ e, of 42 diatomic molecules covering a force constant range of factor 30. The parallel to combining rules for repulsive potentials in solid-state and atomic physics is shown, and the reasons for the failure of previous attempts to obtain transferable force constant increments are examined.