Effect of varying the 1-4 intramolecular scaling factor in atomistic simulations of long-chain N-alkanes with the OPLS-AA model.

A comprehensive molecular dynamics simulation study of n-alkanes using the optimized potential for liquid simulation with all-atoms (OPLS-AA) force field at ambient condition has been performed. Our results indicate that while simulations with the OPLS-AA force field accurately predict the liquid state mass density for n-alkanes with carbon number equal or less than 10, for n-alkanes with carbon number equal or exceeding 12, the OPLS-AA force field with the standard scaling factor for the 1-4 intramolecular Van der Waals and electrostatic interaction gives rise to a quasi-crystalline structure. We found that accurate predictions of the liquid state properties are obtained by successively reducing the aforementioned scaling factor for each increase of the carbon number beyond n-dodecane. To better understand the effects of reducing the scaling factor, its influence on the torsion potential profile, and the corresponding gauche-trans conformer distribution, heat of vaporization, melting point, and self-diffusion coefficient for n-dodecane were investigated. This relatively simple procedure enables more accurate predictions of the thermo-physical properties of longer n-alkanes.