Comparison of Explicit and United Atom Models for Alkane Chains Physisorbed on α-Al2O3 (0001)

Abstract

Explicit atom (EA) and united atom (UA) models are compared for the description of the structure and dynamics of an alkane film adsorbed on an Al-terminated α-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 from the first to reproduce the EA octane adsorption energy. The appropriate scale factor is found to be sensitive to relaxation of the α-Al2O3 surface. The three models are compared in constant NVT molecular dynamics simulations of an octane monolayer (21 molecules) adsorbed on an 80% relaxed α-Al2O3 (0001) surface. Overall, the UA models reproduce the EA structural predictions well, the primary difference being an underestimation of the average gauche angle and overestimation of the trans fraction in the UA simulations. The agreement between dynamic predictions is less satisfactory; the scaled UA model reproduces the EA trans ↔ gauche interconversion rate reasonably well, but fails to reproduce the overall rotational or translational dynamics. UA models must be used with caution when applied to alkane dynamics on a corrugated surface.