Novel configurational-bias Monte Carlo method for branched molecules. Transferable potentials for phase equilibria. 2. United-atom description of branched alkanes

A new generalization of the configurational-bias Monte Carlo method is presented which avoids the problems inherent in a Boltzmann rejection scheme for sequentially generating bond bending and torsional angles. The TraPPE-UA (transferable potentials for phase equilibria united-atom) force field is extended to include Lennard-Jones interaction parameters for methine and quaternary carbon groups by fitting to critical temperatures and saturated liquid densities of branched alkanes. Configurational-bias Monte Carlo simulations in the Gibbs ensemble were carried out to determine the vapor-liquid coexistence curves (VLCC) for six alkane isomers with four to eight carbons. Results are presented for two united-atom alkane force fields: PRF [Poncela, et al. Mol. Phys. 1997, 91, 189] and TraPPE-UA. Standard-state specific densities for the TraPPE-UA model were studied by simulations in the isobaric-isothermal ensemble. It is found that a single set of methyl, methylene, methine, and quaternary carbon parameters gives a reasonable description of the fluid phases of all alkanes with two or more carbon atoms. Whereas the size of the united atoms increases with increasing number of hydrogens for the PRF force field, it is demonstrated here that the opposite trend yields a better fit to the experimental VLCC data. The TraPPE-UA force field underpredicts the magnitude of the experimental second virial coefficients, while PRF gives good second virial coefficients but does not perform satisfactorily for the VLCC. As is also seen for the normal alkanes, the TraPPE-UA force field shows small, systematic deviations from the experimental saturated vapor pressures and densities for all molecules studied, and it slightly overpredicts the critical temperatures of the larger branched alkanes.