Coulombic nonbonded interatomic potential functions derived from crystal-lattice vibrational frequencies in hydrocarbons

The method of calculation of crystal-lattice normal-mode frequencies of hydrocarbons was sharpened to include bond foreshortening of C-H bonds, dynamic derivatives, and atomic point charges. A test of the method was made with 118 observed structural parameters from 18 aromatic and saturated hydrocarbon crystal structures, and 58 observed crystal frequencies from five aromatic hydrocarbon crystal structures. The use of dynamic derivatives significantly improved the fit to the observed frequencies. The use of atomic point charges also significantly improved the fit to the observed frequencies; the optimum values found for the point charges were essentially identical to the optimum values obtained from structural data alone. The direct parameter-fit method, although giving reasonable results for the structural parameters, was found not to transfer well to the calculation of lattice frequencies. The force-fit method gave significantly better results for the lattice frequencies. The final optimum (exp-6-1) nonbonded interatomic potential functions derived from the combined structural and vibrational data were very similar to the functions derived from the structural data alone.