Embedded diatomics-in-molecules potential energy function for methyl radical and methane on nickel surfaces

We have developed a potential energy function for the interaction of H, CH₃, and CH₄ with crystalline Ni surfaces and the dissociation reaction of CH₄ on such surfaces. The potential is based on the embedded diatomics-in-molecules (EDIM) formalism, and it involves mixing the semiempirical diatomics-in-molecules (DIM) valence bond method for the covalent part of the system (CH₄) with the embedded atom method (EAM) for the metal. A new molecular modeling technique is developed to accomplish this, and it employs a physically motivated mapping of electron density into an effective interatomic distance. The EDIM potential energy surface is calibrated against experimental findings and ab initio electronic structure calculations for CH₃ and CH₄ adsorbed on Ni(111), and it reproduces the best available binding geometries, energies, and frequencies quite well. Based on the behavior of the methyl torsion mode for CH₃ in the 3-fold hollow site, our results indicate that the minimum energy orientation for this site has the H atoms staggered with respect to the three Ni atoms that form the hollow site. This differs from the prediction of previous calculations, and we present a possible explanation for this discrepancy. Finally, we calculated the barrier height for dissociative chemisorption of methane to produce CH₃ and H, and we obtained 13.8 kcal/mol, which is within the range of estimates obtained from experiment.