Solid-Fluid Equilibria of Atoms with Soft Repulsive and Short-Range Cohesive Interactions.

We report molecular simulations using a relatively soft interatomic potential to determine the nature of melting under extreme conditions. At temperatures and pressures above the triple point of normal materials, the density range of two-phase solid and fluid equilibria is bounded by freezing and melting curves. We address the unresolved issue of the termination of these boundaries, i.e., whether the melting curve of a solid terminates in a critical point, exhibits a maximum, goes to an asymptotic limit, or continues indefinitely. Significantly, we observe a negative change in volume upon melting at high pressures, which is normally observed only for water. We provide unequivocal evidence that the densities of the meeting and freezing lines can merge at a melting temperature maximum point. This could be a general feature of "soft" atomic fluids at extreme pressures.

New Order-Parameter-Based Method for Characterizing Radiation Damage in Amorphous Materials.

We present a new method of characterizing damage arising from α-recoil cascades in amorphous materials including glasses. The approach taken is topological, yielding information on atom connectivity and utilizing complete sets of orthogonal functions (spherical harmonics and Hermite functions) to compute order parameters. The utility of our new approach is demonstrated by first applying it to models of radiation-damaged crystalline zircon, enabling validation against the standard defect counting method (Wigner-Seitz). We then apply it to a simple model of a glass, obtained by supercooling a Lennard-Jones liquid, for which defect counting methods are inapplicable. The method shows great promise for use in characterizing damage in more complicated glasses, particularly those of interest in the immobilization of nuclear waste, and when used in conjunction with nonequilibrium computer simulation could be a powerful tool to elucidate experimental data on the radiation tolerance of such wasteforms.

Role of Microstructure and Surface Defects on the Dissolution Kinetics of CeO2, a UO2 Fuel Analogue.

The release of radionuclides from spent fuel in a geological disposal facility is controlled by the surface mediated dissolution of UO2 in groundwater. In this study we investigate the influence of reactive surface sites on the dissolution of a synthesized CeO2 analogue for UO2 fuel. Dissolution was performed on the following: CeO2 annealed at high temperature, which eliminated intrinsic surface defects (point defects and dislocations); CeO2-x annealed in inert and reducing atmospheres to induce oxygen vacancy defects and on crushed CeO2 particles of different size fractions. BET surface area measurements were used as an indicator of reactive surface site concentration. Cerium stoichiometry, determined using X-ray Photoelectron Spectroscopy (XPS) and supported by X-ray Diffraction (XRD) analysis, was used to determine oxygen vacancy concentration. Upon dissolution in nitric acid medium at 90 °C, a quantifiable relationship was established between the concentration of high energy surface sites and CeO2 dissolution rate; the greater the proportion of intrinsic defects and oxygen vacancies, the higher the dissolution rate. Dissolution of oxygen vacancy-containing CeO2-x gave rise to rates that were an order of magnitude greater than for CeO2 with fewer oxygen vacancies. While enhanced solubility of Ce(3+) influenced the dissolution, it was shown that replacement of vacancy sites by oxygen significantly affected the dissolution mechanism due to changes in the lattice volume and strain upon dissolution and concurrent grain boundary decohesion. These results highlight the significant influence of defect sites and grain boundaries on the dissolution kinetics of UO2 fuel analogues and reduce uncertainty in the long term performance of spent fuel in geological disposal.

Order parameter and connectivity topology analysis of crystalline ceramics for nuclear waste immobilization.

We apply bond order and topological methods to the problem of analysing the results of radiation damage cascade simulations in ceramics. Both modified Steinhardt local order and connectivity topology analysis techniques provide results that are both translationally and rotationally invariant and which do not rely on a particular choice of a reference structure. We illustrate the methods with new analyses of molecular dynamics simulations of single cascades in the pyrochlores Gd(2)Ti(2)O(7) and Gd(2)Zr(2)O(7) similar to those reported previously (Todorov et al 2006 J. Phys.: Condens. Matter 18 2217). Results from the Steinhardt and topology analyses are consistent, while often providing complementary information, since the Steinhardt parameters are sensitive to changes in angular arrangement even when the overall topological connectivity is fixed. During the highly non-equilibrium conditions at the start of the cascade, both techniques reveal significant localized transient structural changes and variation in the cation connectivity. After a few picoseconds, the connectivity is largely fixed, while the order parameters continue to change. In the zirconate there is a shift to the anion disordered system while in the titanate there is substantial reversion and healing back to the parent pyrochlore structure.

Shock-wave compression and Joule-Thomson expansion.

Structurally stable atomistic one-dimensional shock waves have long been simulated by injecting fresh cool particles and extracting old hot particles at opposite ends of a simulation box. The resulting shock profiles demonstrate tensor temperature, Txx≠Tyy and Maxwell's delayed response, with stress lagging strain rate and heat flux lagging temperature gradient. Here this same geometry, supplemented by a short-ranged external "plug" field, is used to simulate steady Joule-Kelvin throttling flow of hot dense fluid through a porous plug, producing a dilute and cooler product fluid.

The Development of a Classical Force Field To Determine the Selectivity of an Aqueous Fe(3+)-EDA Complex for TcO4(-) and SO4(2.).

A classical force field has been developed in order to investigate the selective exchange of oxyanions (TcO4(-) vs SO4(2-)) with other ligands (H2O, Cl(-)) to an aqueous Fe(3+)-ethylenediamine (EDA) complex. Potentials of mean force for a range of exchange reactions were generated using umbrella sampling and classical molecular dynamics simulations in order to calculate the affinity of each oxyanion for the Fe(3+)-EDA complex in aqueous solution. In order to accurately introduce a degree of specificity for the interaction of Fe(3+) with each ligand type, force field parameters were tuned to match the results of density functional theory calculations. Preferential exchange of H2O, Cl(-), and SO4(2-) for TcO4(-) via an interchange mechanism is observed, in agreement with experimental observations. Both the relative solvation entropies and enthalpies of the anions were found to be critically important factors governing the magnitude of the observed selectivities. These results have important implications for the design and modeling of functionalized materials for the remediation of land contaminated with radioactive (99)Tc.

Computer simulation of the role of torsional flexibility on mass and momentum transport for a series of linear alkanes.

We present the results obtained from a systematic equilibrium molecular dynamics study of the effect of torsional flexibility on the diffusion and viscosity of a series of linear alkanes. To make unambiguous comparisons between molecules with torsional flexibility and those without, we use the frozen distribution sampling (FDS) method introduced by Travis et al. [J. Chem. Phys. 98, 1524 (1993); J. Chem. Phys. 102, 2174 (1995)] but modified and updated for increased efficiency. We first demonstrate comprehensively that FDS guarantees corresponding thermodynamic states. We then show that removal of torsional flexibility results in a significant lowering of the diffusion coefficient (and corresponding increase in shear viscosity) and furthermore that this effect increases with increasing chain length. The results are discussed in terms of the possible mechanism giving rise to this dynamic coupling phenomenon.

Dynamic simulations of colloids by core-modified dissipative particle dynamics.

We develop a core-modified dissipative particle dynamics model of colloidal systems which includes an extra term to counteract depletion forces. Results are presented covering the full range of volume fractions. Radial distribution functions for the suspending fluid are shown to change significantly as the volume fraction of colloid increases. Equilibrium results for the long-time diffusion coefficient behave as expected, but the short-time coefficient is anomalous. The form of the equilibrium stress correlation functions is discussed and the derived Green-Kubo viscosities are compared with expected semiempirical forms. For nonequilibrium shear-field simulations we find that the system temperature is not adequately controlled by the dissipative particle dynamics (DPD) thermostat alone. Results using three alternative auxiliary thermostats are compared; a naive choice leading to a string phase at high shear rate. Using a thermostat based on relative particle velocities, the model reproduced the four classical regions of colloid rheology: a first Newtonian plateau, a shear-thinning region, a second Newtonian plateau, and finally a shear-thickening region at high strain rate. The most unexpected result of this exercise is that the core-modified DPD model without auxiliary thermostat almost exactly follows the same curve despite recording a temperature increase of a factor approximately 2.5 over the range.

Configurational temperature control for atomic and molecular systems.

A new configurational temperature thermostat suitable for molecules with holonomic constraints is derived. This thermostat has a simple set of motion equations, can generate the canonical ensemble in both position and momentum space, acts homogeneously through the spatial coordinates, and does not intrinsically violate the constraints. Our new configurational thermostat is closely related to the kinetic temperature Nosé-Hoover thermostat with feedback coupled to the position variables via a term proportional to the net molecular force. We validate the thermostat by comparing equilibrium static and dynamic quantities for a fluid of n-decane molecules under configurational and kinetic temperature control. Practical aspects concerning the implementation of the new thermostat in a MOLECULAR DYNAMICS code and the potential applications are discussed.

Parameterization of the nonlocal viscosity kernel for an atomic fluid.

In this paper we present results for the wave-vector dependent shear viscosity for a model atomic fluid with short ranged repulsive interactions computed by molecular dynamics simulations. It is shown that the data can be fitted to two different simple functional forms over a large density range, namely, a function composed of two Gaussian terms and a Lorentzian type function with a variable wave-vector exponent. The parameters of both functional forms are found to obey simple density dependencies. While the first functional form has the advantage that the inverse Fourier transform can be found analytically, the Lorentzian type function fits the wave-vector dependence better over the range of wave vectors and densities studied here. The results show that the real space viscosity kernel has a width of 2 to 3 atomic diameters. This means that the generalized hydrodynamic constitutive relation is required if the strain rate varies significantly over this distance, a situation commonly encountered for nanofluidic flows.

New parametrization method for dissipative particle dynamics.

We introduce an improved method of parametrizing the Groot-Warren version of dissipative particle dynamics (DPD) by exploiting a correspondence between DPD and Scatchard-Hildebrand regular solution theory. The new parametrization scheme widens the realm of applicability of DPD by first removing the restriction of equal repulsive interactions between like beads, and second, by relating all conservative interactions between beads directly to cohesive energy densities. We establish the correspondence by deriving an expression for the Helmoltz free energy of mixing, obtaining a heat of mixing which is exactly the same form as that for a regular mixture (quadratic in the volume fraction) and an entropy of mixing which reduces to the ideal entropy of mixing for equal molar volumes. We equate the conservative interaction parameters in the DPD force law to the cohesive energy densities of the pure fluids, providing an alternative method of calculating the self-interaction parameters as well as a route to the cross interaction parameter. We validate the new parametrization by modeling the binary system SnI(4)SiCl(4), which displays liquid-liquid coexistence below an upper critical solution temperature around 140 degrees C. A series of DPD simulations were conducted at a set of temperatures ranging from 0 degrees C to above the experimental upper critical solution temperature using conservative parameters based on extrapolated experimental data. These simulations can be regarded as being equivalent to a quench from a high temperature to a lower one at constant volume. Our simulations recover the expected phase behavior ranging from solid-liquid coexistence to liquid-liquid coexistence and eventually leading to a homogeneous single phase system. The results yield a binodal curve in close agreement with the one predicted using regular solution theory, but, significantly, in closer agreement with actual solubility measurements.

Thermodynamic parameters of bonds in glassy materials from viscosity-temperature relationships.

Doremus's model of viscosity assumes that viscous flow in amorphous materials is mediated by broken bonds (configurons). The resulting equation contains four coefficients, which are directly related to the entropies and enthalpies of formation and motion of the configurons. Thus by fitting this viscosity equation to experimental viscosity data these enthalpy and entropy terms can be obtained. The non-linear nature of the equation obtained means that the fitting process is non-trivial. A genetic algorithm based approach has been developed to fit the equation to experimental viscosity data for a number of glassy materials, including SiO2, GeO2, B2O3, anorthite, diopside, xNa2O-(1-x)SiO2, xPbO-(1-x)SiO2, soda-lime-silica glasses, salol, and α-phenyl-o-cresol. Excellent fits of the equation to the viscosity data were obtained over the entire temperature range. The fitting parameters were used to quantitatively determine the enthalpies and entropies of formation and motion of configurons in the analysed systems and the activation energies for flow at high and low temperatures as well as fragility ratios using the Doremus criterion for fragility. A direct anti-correlation between fragility ratio and configuron percolation threshold, which determines the glass transition temperature in the analysed materials, was found.

Effect of solvation and confinement on the trans-gauche isomerization reaction in n-butane.

The effect of solvation and confinement on the conformational equilibria and kinetics of n-butane is examined using molecular dynamics simulations of the bulk and confined fluids and compared to appropriately chosen reference states. Clear evidence for a solvent shift of the preferred conformation in bulk n-butane is found. At a temperature of 292 K and a density of 6.05 nm-3 a small solvent shift in favor of gauche is observed (similar to previously reported values), and the shift increases substantially with an increase in density to 8.28 nm-3. The rate of torsional interconversion from the trans to the gauche state, calculated using the relaxation function method, was found to increase with increasing temperature and density. The rate constants kTG and kGT have an Arrhenius temperature dependence yielding activation energies significantly lower than the trans-gauche and gauche-trans barrier heights in the torsional potential for a free molecule, depending on the density. In the confined phase, we considered the same densities as simulated in the bulk phase, and for four different values of the physical pore width (approximately 1.5-4.0 nm). At the high density, we find that the position of the trans-gauche equilibrium is displaced towards excess trans compared with the bulk phase, reflecting the confinement and interactions of the molecules with the pore wall. The isomerization rate is found to decrease with decreasing pore width. Again, we find that the kinetics obeys an Arrhenius rate law and the activation energy for the trans-gauche and gauche-trans interconversions is slightly smaller than that of the bulk fluid at the same density.

Configurational constant pressure molecular dynamics.

We propose two new algorithms for generating isothermal-isobaric molecular dynamics. The algorithms are based on an extended phase space dynamics where two extra degrees of freedom, representing the thermostat and the barostat, are included. These new methods adopt a totally different approach towards molecular dynamics simulation in the isothermal-isobaric ensemble. They are fully configurational in the sense that only the particle positions are required in the control of the system temperature and pressure. Following on from the works of Delhommelle and Evans [Mol. Phys., 99, 1825 (2001)] and of Braga and Travis [J. Chem. Phys., 123, 134101 (2005)] concerning configurational canonical dynamics, these new algorithms can be seen as a natural extension to the isothermal-isobaric ensemble. We have validated both of our new configurational isothermal-isobaric schemes by conducting molecular dynamics simulations of a Lennard-Jones fluid and comparing the static and dynamic properties for a single state point. We find that both schemes generate similar results compared with schemes which use kinetic temperature and pressure control. We have also monitored the response of the system to a series of isothermal compressions and isobaric quenches. We find that the configurational schemes performed at least as well as the kinetic based scheme in bringing the system temperature and pressure into line with the set point values of these variables. These new methods will potentially play a significant role in simulations where the calculation of the kinetic temperature and pressure can be problematic. A well known example resides in the field of nonequilibrium simulations where the kinetic temperature and pressure require a knowledge of the streaming velocity of the fluid in order to calculate the true peculiar velocities (or momenta) that enter into their definitions. These are completely avoided by using our configurational thermostats and barostats, since these are independent of momenta. By extending the analysis of Kusnezov et al. [Ann. Phys., 204, 155 (1990)] in order to derive a set of generalized Nose-Hoover equations of motion which can generate isothermal-isobaric dynamics in a number of different ways, we are able to show that both of our new configurational barostats and Hoover's kinetic isothermal-isobaric scheme are special cases of this more general set of equations. This generalization can be very powerful in generating constant pressure dynamics for a variety of systems.

A configurational temperature Nosé-Hoover thermostat.

We propose two new thermostats which can be employed in computer simulations to ensure that two different variants of the configurational temperature fluctuate around their equilibrium values. These new thermostats differ from one previously introduced by Delhommelle and Evans [Mol. Phys. 99, 1825 (2001)] in several important ways. First, our thermostats are derived in the same spirit as the Nosé-Hoover thermostat and therefore generate the canonical phase-space distribution. Second, our thermostats involve simpler equations of motion, which do not involve spatial gradients of the configurational temperature. They do not suffer from problems stemming from stiff equations of motion and furthermore, in large temperature perturbation simulations, the measured temperature follows the set-point temperature without any overshoot, and with good damping of oscillations. We show that both of our configurational thermostats are special cases of a more general set of Nosé-Hoover equations proposed by Kusnezov et al. [Ann. Phys. 204, 155 (1990)]. The new thermostats are expected to be highly useful in nonequilibrium simulations, particularly those characterized by spatial inhomogeneities. They should also find applicability in simulations involving large changes in temperature over small time scales, such as temperature quench molecular dynamics and radiation damage modeling.

A molecular dynamics study of nitric oxide in water: diffusion and structure.

We present molecular dynamics simulations of the diffusion coefficients and structure of water-nitric oxide mixtures at ambient (298 K) and in vivo (310 K) conditions. A two-site rigid-body molecular model with partial charges and a Lennard-Jones potential on both sites is proposed for nitric oxide and used in conjunction with the extended simple point-charge model for liquid water in our simulations. The diffusion coefficients obtained from the simulations are in good agreement with experimental data. The results from intermolecular partial pair functions show that under these thermodynamic conditions, the existence of nitric oxide in liquid water has little impact on the structure of water and the tendency to form H bonds between water molecules. We also find that it is unlikely that H bonds form between the hydrogen atoms in water and either the nitrogen or the oxygen atom on the nitric oxide at the temperatures and densities examined in this study. This study suggests that in low concentrations nitric oxide molecules exist as free molecules in liquid water rather than forming complexes with water molecules.

Viscosity of confined inhomogeneous nonequilibrium fluids.

We use the nonlocal linear hydrodynamic constitutive model, proposed by Evans and Morriss [Statistical Mechanics of Nonequilibrium Liquids (Academic, London, 1990)], for computing an effective spatially dependent shear viscosity of inhomogeneous nonequilibrium fluids. The model is applied to a simple atomic fluid undergoing planar Poiseuille flow in a confined channel of several atomic diameters width. We compare the spatially dependent viscosity with a local generalization of Newton's law of viscosity and the Navier-Stokes viscosity, both of which are known to suffer extreme inaccuracies for highly inhomogeneous systems. The nonlocal constitutive model calculates effective position dependent viscosities that are free from the notorious singularities experienced by applying the commonly used local constitutive model. It is simple, general, and has widespread applicability in nanofluidics where experimental measurement of position dependent transport coefficients is currently inaccessible. In principle the method can be used to predict approximate flow profiles of any arbitrary inhomogeneous system. We demonstrate this by predicting the flow profile for a simple fluid undergoing planar Couette flow in a confined channel of several atomic diameters width.