CALCULATION OF CHEMICAL-POTENTIAL FOR STRUCTURED MOLECULES USING OSMOTIC MOLECULAR-DYNAMICS SIMULATIONS

A previously reported osmotic molecular dynamics (OMD) method for calc ulation of chemical potential is extended to structured molecules. The viability of the method is confirmed by the agreement between chemica l potential values obtained directly from the OMD simulations and thos e obtained using a thermodynamic identity and the simulated pressures. The validity of the method was also supported by the agreement of den sities 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 equ ations of state over large temperature and density ranges are availabl e in the literature. Values of the model n-butane chemical potential c omputed 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 valu es for structured molecules at high densities, common to traditional m ethods that are based on artificial particle insertions, are alleviate d by OMD simulations, because the chemical potential is calculated dir ectly from the mechanical variables of a system in osmotic equilibrium . (C) 1997 Elsevier Science B.V.