Thomas A. Weber

Packing Structures and Transitions in Liquids and Solids

Classification of potential energy minima—mechanically stable molecular packings—offers a unifying principle for understanding condensed phase properties. This approach permits identification of an inherent structure in liquids that is normally obscured by thermal motions. Melting and freezing occur through characteristic sequences of molecular packings, and a defect-softening phenomenon underlies the fact that they are thermodynamically first order. The topological distribution of feasible transitions between contiguous potential minima explains glass transitions and associated relaxation behavior.

Simulation of n‐butane using a skeletal alkane model

A simulation of a fluid n‐butane system has been carried out using a straight‐chain skeletal model. The calculations reported here involve fluid densities from 288.80 to 721.99 kg/m3 at several distinct temperatures. In the course of this investigation the linear self‐diffusion constant, the rate of torsional gauche/trans relaxation and rotational tumbling of the fluid have been studied. This model shows a high degree of cooperativity between the molecular vibration and rotation, and the bulk fluid motion.

Inherent pair correlation in simple liquids

This paper is dedicated to the proposition that liquids possess an inherent packing structure which is determined by their collection of potential energy minima. To reveal the inherent structure in a given thermodynamic state, it is necessary to subject the dynamical system to steepest‐descent ‘‘quenches’’ that remove thermal motion and distortion and leave the system in the nearest mechanically stable arrangement. Such a program has been carried out via molecular dynamics simulation on an argon‐like system containing 108 atoms. Two thermodynamic states at the same reduced density (ρ*=1.0) were considered, one just above the melting temperature T*m and one at approximately 3.5 T*m . Although the pair correlation functions g(r) in these thermodynamic states differed considerably, those produced by the corresponding sets of quenches gq(r) were virtually identical. The implied inherent structure common to both states appears to be best described in terms of highly defective face‐centered‐cubic crystalline co...

Molecular dynamics simulation for chemically reactive substances. Fluorine

Molecular dynamics computer simulation has been utilized to study physical and chemical properties of the highly reactive element fluorine in its fluid phases. The underlying model approximates the energy of the ground electronic state for an arbitrary collection of fluorine atoms with a combination of two and three atom interactions. The classical simulation employed 1000 atoms subject to periodic boundary conditions. Diatomic molecules spontaneously form and are stable at low temperatures, but dissociation and atom exchange reactions occur at high temperatures. Steepest‐descent quenching on the potential energy hypersurface reveals the presence of a temperature‐independent inherent structure for the low‐temperature undissociated liquid. Dissociation is found to be strongly enhanced at high density owing to relatively strong solvation by diatomics of chemically unbonded fluorine atoms. Slow cooling of the fluid from well above the critical temperature, at one‐eighth of the triple‐point density, produced ...

Brownian dynamics study of polymer conformational transitions

Conformational transitions of a macromolecule have been studied by computer simulation of the Brownian dynamics of a polymer chain. An activation energy equal to about one barrier height has been found. However, there is a great deal of cooperativity of transitions especially between bonds which are second nearest neighbors.

Point defects in bcc crystals: Structures, transition kinetics, and melting implications

Structures corresponding to various potential energy minima have been examined for a classical model whose pair interactions produce a body‐centered‐cubic crystalline ground state. The method used is molecular dynamics computer simulation for 128 particles with frequent steepest‐descent mapping onto nearby minima. The elementary structural excitation out of the crystalline absolute minimum is creation of a vacancy, split‐interstitial defect pair. This excitation process upon repetition shows a defect softening of the medium. Transition states (saddle points) have been located for some pairs of neighboring minima, and vibrational modes have been calculated for minima and for transition states. A simple melting theory based on these observations is proposed which satisfactorily describes the model’s first‐order melting behavior.

Erratum: Study of melting and freezing in the Gaussian core model by molecular dynamics simulation

Molecular dynamics calculations have been carried out to establish quantitative properties of the Gaussian core model near its crystal–fluid transition. Two densities have been considered, for both of which the stable crystal structure at absolute zero is body‐centered cubic. Spontaneous melting and freezing events were observed at both densities. Annealing of defective crystalline phases and formation of amorphous ’’glassy’’ structures have been induced. Properties for the model at equilibrium display some surprising ’’waterlike’’ anomalies, including negative volume of melting, negative thermal expansion in the fluid, and increase in rate of self‐diffusion as the system is compressed.

Chemical reactions in liquids: Molecular dynamics simulation for sulfur

A combination of two‐atom and three‐atom interactions has been selected to represent the structural chemistry of sulfur. This model potential exhibits divalency (bond saturation) and leads to the known preference for Sn molecules to form puckered ring structures. Using this representation of the interactions, molecular dynamics calculations have been performed for 1000 sulfur atoms at the experimental liquid density. Short‐range order has been calculated for the low‐temperature liquid consisting of S8 cyclic molecules, and agrees qualitatively with the (imprecise) available measurements. At elevated temperatures the cyclic S8 molecules in the simulation begin to break open, and their subsequent chemical reactions yield primarily linear polymeric species. A metastable reaction intermediate in the polymerization process has been identified, a ‘‘tadpole’’ consisting of a diradical chain attached weakly to an S8 ring.

Dynamical branching during fluorination of the dimerized Si(100) surface: A molecular dynamics study

Collections of classical trajectories have been numerically generated for individual F2 molecules impinging at normal incidence on a Si(100) surface at 0 K dimerized in a p(2×1) pattern. A linear combination of two‐atom and three‐atom interaction functions represents the potential energy. Trajectories fall into four categories: (a) non‐reactive F2 rebound, (b) monofluorination at a surface dangling bond with energetic expulsion into the vacuum of the remaining F atom, (c) difluorination of a pair of dangling bonds, and (d) monofluorination with retention of the second F in a weakly bound Si–F⋅⋅⋅F surface complex. Surface patterns for difluorination, (c), indicate absence of surface diffusion during this mode of chemisorption. Increasing either the translational kinetic energy or the vibrational excitation of the incident F2 appears to enhance its surface reactivity.

Relaxation of a n‐octane fluid

This paper reports the results of a series of molecular dynamics calculations on a n‐octane fluid at the normal density and several temperatures. The torsional interconversion between gauche and trans bonds has been investigated as a function of bond position and temperature. It is found that intermolecular interactions retard trans/gauche relaxation and appear to influence the trans/gauche ratio of the fluid.