Calculation of chemical potential for structured molecules using osmotic molecular dynamics simulations

A previously reported osmotic molecular dynamics (OMD) method for calculation of chemical potential is extended to structured molecules. The viability of the method is confirmed by the agreement between chemical potential values obtained directly from the OMD simulations and those obtained using a thermodynamic identity and the simulated pressures. The validity of the method was also supported by the agreement of densities and pressures for the fluid in the central portion of the half-cell with the same properties computed via standard molecular dynamics (MD) simulations. A site-site model of four equivalent Lennard-Jones (LJ) potentials was used to represent n-butane, for which accurate equations of state over large temperature and density ranges are available in the literature. Values of the model n-butane chemical potential computed from OMD simulations agreed well with the values obtained from these accurate n-butane equations of state over the whole temperature-density domain studied. Problems in obtaining chemical potential values for structured molecules at high densities, common to traditional methods that are based on artificial particle insertions, are alleviated by OMD simulations, because the chemical potential is calculated directly from the mechanical variables of a system in osmotic equilibrium.