Comparison of explicit and united atom models for alkane chains physisorbed on alpha-Al2O3 (0001)

Explicit atom (EA) and united atom (UA) models are compared for the descrip tion of the structure and dynamics of an alkane film adsorbed on an Al-term inated alpha-Al2O3 (0001) surface. Ab initio calculations for an Al2O3 ... CH4 cluster are used to parameterize the EA model. One UA model is derived by explicit averaging of the EA potential, while the second is rescaled fro m the first to reproduce the EA octane adsorption energy. The appropriate s cale factor is found to be sensitive to relaxation of the alpha-Al2O3 surfa ce. The three models are compared in constant NVT molecular dynamics simula tions of an octane monolayer (21 molecules) adsorbed on an 80% relaxed alph a-Al2O3 (0001) surface. Overall, the UA models reproduce the EA structural predictions well, the primary difference being an underestimation of the av erage gauche angle and overestimation of the trans fraction in the UA simul ations. The agreement between dynamic predictions is less satisfactory; the scaled UA model reproduces the EA trans <-> gauche interconversion rate re asonably well, but fails to reproduce the overall rotational or translation al dynamics. UA models must be used with caution when applied to alkane dyn amics on a corrugated surface.