From dimers to the solid-state: Distributed intermolecular force-fields for pyridine

A.A. thanks A.W.E. financial support through the EngDoc studentship from M3S Centre for Doctoral Training (EPSRC Grant No. EP/G036675/1). General computational infrastructure used is developed under No. EPSRC EP/K039229/1

Quasi Harmonic Lattice Dynamics and Molecular Dynamics calculations for the Lennard-Jones solids

We present Molecular Dynamics (MD), Quasi Harmonic Lattice Dynamics (QHLD) and Energy Minimization (EM) calculations for the crystal structure of Ne, Ar, Kr and Xe as a function of pressure and temperature. New Lennard-Jones (LJ) parameters are obtained for Ne, Kr and Xe to reproduce the experimental pressure dependence of the density. We employ a simple method which combines results of QHLD and MD calculations to achieve densities in good agreement with experiment from 0 K to melting. Melting is discussed in connection with intrinsic instability of the solid as given by the QHLD approximation. (See http://www.fci.unibo.it/~valle for related papers)Comment: 7 pages, 5 figures, REVte

To wet or not to wet: that is the question

Wetting transitions have been predicted and observed to occur for various combinations of fluids and surfaces. This paper describes the origin of such transitions, for liquid films on solid surfaces, in terms of the gas-surface interaction potentials V(r), which depend on the specific adsorption system. The transitions of light inert gases and H2 molecules on alkali metal surfaces have been explored extensively and are relatively well understood in terms of the least attractive adsorption interactions in nature. Much less thoroughly investigated are wetting transitions of Hg, water, heavy inert gases and other molecular films. The basic idea is that nonwetting occurs, for energetic reasons, if the adsorption potential's well-depth D is smaller than, or comparable to, the well-depth of the adsorbate-adsorbate mutual interaction. At the wetting temperature, Tw, the transition to wetting occurs, for entropic reasons, when the liquid's surface tension is sufficiently small that the free energy cost in forming a thick film is sufficiently compensated by the fluid- surface interaction energy. Guidelines useful for exploring wetting transitions of other systems are analyzed, in terms of generic criteria involving the "simple model", which yields results in terms of gas-surface interaction parameters and thermodynamic properties of the bulk adsorbate.Comment: Article accepted for publication in J. Low Temp. Phy

Water-graphite interaction and behavior of water near the graphite surface

Based on MP2 level electronic structure calculations of the binding energy and lowest energy conformation of water−acene complexes [Feller, D.; Jordan, K. D. J. Phys. Chem. A 2000, 104 (44), 9971−9975], three water−graphite model potentials are suggested and tested in grand canonical Monte Carlo simulations of the behavior of water confined between two parallel graphite sheets. It is shown that the thermodynamics and structure of the water−graphite interfacial region are extremely sensitive to the range and orientation dependence of the model potential. This casts doubt on the results of previous molecular dynamics simulations using orientation-independent potentials and standard atomistic force fields. All of the three suggested potentials predict that the water monolayer compressed between two parallel graphite surfaces does not experience capillary evaporation and offers only slight resistance to shear. This explains why water can serve as a lubricant in the friction of graphitic carbons

Computer simulations of water-mediated force between phospholipid membranes

The knowledge of forces operating between phospholipid bilayer membranes in water and aqueous solutions is a prerequisite for understanding membrane-membrane coupling phenomena such as stacking, adhesion, and fusion. This explains the substantial efforts undertaken in the last two decades to measure and rationalize the intermembrane force as a function of separation, with an emphasis on short-range repulsion. Despite considerable progress in experimental measurements, the interpretation of the force-distance curves in terms of physically distinct force components involves serious difficulties because the experiment provides only the total magnitude of the force. All this imparts importance to computer simulations, that allow direct evaluation of the intermembrane force and its components through the respective ensemble averages. In this paper we briefly review these computer simulations, as well as some relevant studies. The simulation results are discussed in the context of the existing theories of the intermembrane forces