Transferable potentials for phase equilibria. 1. united-atom description of n-alkanes

An extension of the transferable potentials for phase equilibria united-atom (TraPPE-UA) force field to thiol, sulfide and disulfide functionalities and thiophene is presented. In the TraPPE-UA force field, non-bonded interactions are governed a Lennard-Jones plus fixed point charge functional form. Partial charges are determined through a CHELPG analysis of electrostatic potential energy surfaces derived from ab initio calculations at the HF/6-31g+(d,p) level. The Lennard-Jones well depth and size parameters for four new interactions sites, S (thiols), S(sulfides) S(disulfides), and S(thiophene) were determined by fitting simulation data to pure component vapor-equilibrium data for methanethiol, dimethylsulfide, dimethyldisulfide, and thiophene, respectively. Configurational-bias Monte Carlo simulations in the grand canonical ensemble, combined with histogram-reweighting methods were used to calculate the vapor-liquid coexistence curves for methanethiol, ethanethiol, 2-methyl-1-propanethiol, 2-methyl-2-propanethiol, 2-butanethiol, pentanethiol, octanethiol, dimethylsulfide, di-ethylsulfide, ethylmethylsulfide, dimethyldisulfide, diethyldisulfide, and thiophene. Ex-cellent agreement with experiment is achieved, with unsigned errors of less than 1 % for saturated liquid densities and less than 3 % for critical temperatures. Normal boilin