Vapour-liquid coexistence curves of the united-atom and anisotropic united-atom force fields for alkane mixtures

Scaling Laws and Critical Properties for fcc and hcp Metals

The determination of the critical parameters of metals has remained particularly challenging both experimentally, because of the very large temperatures involved, and theoretically, because of the many-body interactions that take place in metals. Moreover, experiments have shown that these systems exhibit an unusually strong asymmetry of their binodal. Recent theoretical work has led to new similarity laws, based on the calculation of the Zeno line and of the underlying Boyle parameters, which provided results for the critical properties of atomic and molecular systems in excellent agreement with experiments. Using the recently developed expanded Wang-Landau (EWL) simulation method, we evaluate the grand-canonical partition function, over a wide range of conditions, for 11 fcc and hcp metals (Ag, Al, Au, Be, Cu, Ir, Ni, Pb, Pd, Pt, and Rh), modeled with a many-body interaction potential. This allows us to calculate the binodal, Zeno line, and Boyle parameters and, in turn, obtain the critical properties for these systems. We also propose two scaling laws for the enthalpy and entropy of vaporization, and identify critical exponents of 0.4 and 1.22 for these two laws, respectively.

Polarizable empirical force field for alkanes based on the classical Drude oscillator model

Recent extensions of potential energy functions used in empirical force field calculations have involved the inclusion of electronic polarizability. To properly include this extension into a potential energy function it is necessary to systematically and rigorously optimize the associated parameters based on model compounds for which extensive experimental data are available. In the present work, optimization of parameters for alkanes in a polarizable empirical force field based on a classical Drude oscillator is presented. Emphasis is placed on the development of parameters for CH3, CH2, and CH moieties that are directly transferable to long chain alkanes, as required for lipids and other biomolecules. It is shown that a variety of quantum mechanical and experimental target data are reproduced by the polarizable model. Notable is the proper treatment of the dielectric constant of pure alkanes by the polarizable force field, a property essential for the accurate treatment of, for example, hydrophobic solvation in lipid bilayers. The present alkane force field will act as the basis for the aliphatic moieties in an extensive empirical force field for biomolecules that includes the explicit treatment of electronic polarizability.