Constant pressure path integral molecular dynamics studies of quantum effects in the liquid state properties of n-alkanes

A computer simulation study of quantum effects in methane, butane, and octane is presented. Each molecular system is examined at three state points in the liquid region using novel extended system, multiple time step, constant pressure, path integral molecular dynamics methodology. In addition, the results of classical calculations are reported to provide a useful reference. Liquid butane is used as a test case on which to compare the predictions of two empirical force fields, CHARMM22 and AMBER95. Comparisons are made to experiment. Briefly, the models predict that quantum effects lead to an increase in molar volume of approximately 2 cm3/mole (i.e., relative to a classical calculation). However, a slight unphysical hydrogen–deuterium isotope effect is, also, observed. This may be caused by an incorrect parametrization of the anisotropy of the potential or by a reduction in the magnitude of the intermolecular induced dipole-induced dipole dispersion coefficient with increasing isotope mass that has not been parametrized in the force fields. In addition, the results show an interesting zero-point energy effect. The intramolecular regions of the radial distribution function exhibit less structure at lower temperatures than at higher temperatures. This is the inverse of the prediction of the model in the classical limit. The quantum effect occurs because the bulk density decreases faster than the intramolecular degrees of freedom lose zero-point energy as temperature increases in the highly harmonic intramolecular potential model employed in the calculations. Nonetheless, the phenomena is not likely to be an artifact and careful experiments could observe it. Finally, the efficiency of the path molecular dynamics methods employed in the studies are demonstrated on both serial and parallel computers.