Extension of Mie Potentials to Branched Alkanes, Alkenes, and Ethers

Transferable united-atom force fields, based on n - 6 Lennard-Jones potentials, are presented for branched alkanes, alkenes and ethers. It is shown that by varying the repulsive exponent the range of the potential can be altered, leading to improved predictions of vapor pressures while also reproducing saturated liquid densities to high accuracy. Histogram-reweighting Monte Carlo simulations in the grand canonical ensemble are used to determine the vapor liquid coexistence curves, vapor pressures, heats of vaporization, and critical points for a variety of pentane and hexane isomers, 1-alkenes (ethane to octane), 2-alkenes (cis-2-butene, trans-2-butene), butadiene, dimethyl ether and diethyl ether.   These data are combined with quantum chemical calculations to illustrate the effect of bonding environment on the transferability of potential parameters.  For all molecules studied, saturated liquid densities and vapor pressures are reproduced to within 1% and 5% of experiment, respectively.