VALIDATION OF A REPARAMETERIZED MM3 NONBONDED FORCE-FIELD FOR HYDROCARBONS - CRYSTAL-LATTICE STUDIES OF C-60 AND C-70 AND ADSORPTION OF HYDROCARBONS ONTO GRAPHITE

Four hydrocarbon van der Waals potentials have been employed to obtain crystal unit cell dimensions and enthalpy sublimation values for two fullerenes, C-60 and C-70 The same force fields have also been employe d in computing adsorption energies between a series of hydrocarbon mol ecules and a model graphite basal plane. The four hydrocarbon force fi elds are MM2, MM3, MM3hc, which is a new reparametrization of MM3 by t he authors in earlier studies, and the potential of Williams and Starr . The MM3 and MM3hc force fields have been shown to offer the best agr eement between the experimental and calculated crystal data for C-60 a nd C-70 In the graphite-adsorbate molecule computations, both charged and uncharged models for the four force fields have been tested to est ablish which model provides the most accurate structural and thermodyn amic results. These calculations indicate that models with no charges are superior despite their deficiencies. For the adsorption of aliphat ic molecules onto graphite, MM3hc yields closer agreement between expe rimental and calculated energies of adsorption than its counterparts, while its predictions for aromatic compounds are slightly less accurat e than the MM2 and MM3 results. All force fields are similar in their predictions of structural arrangements of the adsorbate molecules on g raphite. However, the computed structures disagree with experimental f indings in several cases, underlining the inadequacy of simple analyti cal potential functions for graphite-hydrocarbon interactions.