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The structure of a nanodrop composed of the binary mixture and sitting on the smooth solid surface is examined on the basis of a nonlocal canonical ensemble density functional theory. The molecules...

By combining Monte Carlo simulations and analytical models, we demonstrate and explain how the gas-to-liquid phase transition of colloidal systems confined to a spherical surface depends on the curvature and size of the surface, and on the choice of thermodynamic ensemble. We find that the geometry of the surface affects the shape of the free energy profile and the size of the critical nucleus by altering the perimeter-area ratio of isotropic clusters. Confinement to a smaller spherical surface results in both a lower nucleation barrier and a smaller critical nucleus size. Furthermore, the liquid domain does not grow indefinitely on a sphere. Saturation of the liquid density in the grand canonical ensemble and the depletion of the gas phase in the canonical ensemble lead to a minimum in the free energy profile, with a sharp increase in free energy for additional growth beyond this minimum., 13 pages, 7 figures 1 Revision: In the original arXive submission, Halim Kusumaatmaja's name has been mistakenly left out of the metadata

By combining Gibbs Ensemble Monte Carlo simulations and density functional theory, we investigate the influence of confinement in a slit-shaped carbon pore on the solubility of a supercritical solute gas in a liquid solvent. In the cases studied here, competing adsorption of the solvent and solute determines whether the solubility is enhanced or suppressed for larger pores. We find that the solubility in the confined system is strongly dependent on pore width, and that molecular packing effects are important for small pore widths. In addition, the solubility decreases on increase in the temperature, as for the bulk mixture, but the rate of decrease is greater in the pore due to a decrease in the partial molar enthalpy of the solute in the pore; this effect becomes greater as pore width is decreased. The solubility is increased on increasing the bulk pressure of the gas in equilibrium with the pore, and obeys Henry's law at lower pressures. However, the Henry constant differs significantly from tha...

We present a thermodynamic perturbation theory for fluids composed of N quantum particles of diameter α and mass m contained within a volume V at temperature T, interacting via a pair square-well potential (SW) with energy depth e and range λ. Our approach is based on the exact analogy between the discretised path- integral formalism of quantum mechanics and the partition function of a classical system composed of necklace molecules, introducing a Zwanzig expansion method, using a quantum hard-spheres fluid (QHS) as reference system in order to calculate the perturbation terms for the Helmholtz free energy AQSW of the SW system. These terms are obtained with path-integral Monte Carlo simulations in the canonical ensemble, and analytical results are provided as functions of the inverse temperature β = 1/kT, density ρ* = Nσ3/V, SW range λ and de Broglie thermal wavelength , where h and k are the Planck's and Boltzmann's constants, respectively. Accurate results are obtained for thermodynamic states ...

This work illustrates the application of a three-party approach based on theoretical modelling, molecular dynamic (MD) simulations and available experimental data for describing the phase equilibrium and interfacial properties for the ternary system: carbon dioxide + n-butane + n-decane and its corresponding binary sub-systems at 344.3 K. Specifically, a coarse-grained force field is employed for both theoretical predictions and MD. The interfacial region is described by the square gradient theory where the homogenous Helmholtz energy density contribution is provided by the Statistical Associated Fluid Theory equation of state for potentials of variable range for molecules conformed of segments interacting through the Mie potential (SAFT-VR Mie) and MD simulations in the canonical ensemble where the molecules are represented by a coarse-grained Mie force field. The novelty here is that both the theory and the simulations uniquely share the same underlying intermolecular potentials; hence, the expe...

Liquid-crystal phase equilibria of Lennard-Jones chain fluids and the solubility of a Lennard-Jones gas in the coexisting phases are calculated from Monte Carlo simulations. Direct phase equilibria calculations are performed using an expanded formulation of the Gibbs ensemble. Monomer densities, order parameters, and equilibrium pressures are reported for the coexisting isotropic and nematic phases of: (1) linear Lennard-Jones chains, (2) a partially-flexible Lennard-Jones chain, and (3) a binary mixture of linear Lennard-Jones chains. The effect of chain length is determined by calculating the isotropic-nematic coexistence of linear Lennard-Jones chain fluids made of 8, 10, and 12 segments (8-, 10-, 12-mer). The effect of molecular flexibility on the isotropic-nematic equilibrium is studied for a Lennard-Jones 10-mer chain fluid with one freely-jointed segment at the end of the chain. An isotropic-nematic phase split and fractionation are reported for a binary mixture of linear 7-mer and 12-mer c...

Molecular dynamics simulations of the pyridine N-oxide aqueous solution have been performed in the canonical ensemble macroscopic canonical ensemble (NVT) both in the absence and presence of an external electromagnetic field. It extracts the radial distribution function for each concentration solution, dielectric constant and other information on dielectric properties. Analysing the microscopic dielectric information of the aqueous solution under the static electric field (0–3×109 V/m) and microwave frequencies (2.45G, 0–3×109 V/m), and comparing the dielectric information between the different concentrations and different field strengths, we can get the dielectric properties of two kinds of polar aqueous solution under microwave irradiation. Thus, this project can provide the data of the sample to other correlation studies.

We determine the absolute grand potential Λ along a confined smectic-A branch of a calamitic liquid crystal system enclosed in a slit pore of transverse area A and width L, using the rod–rod Gay–Berne potential and a rod–wall potential favouring perpendicular orientation at the walls. For a confined phase with an integer number of smectic layers sandwiched between the opposite walls, we obtain the excess properties (excess grand potential Λexc, solvation force fs and adsorption Γ) with respect to the bulk phase at the same μ (chemical potential) and T (temperature) state point. While usual thermodynamic integration methods are used along the confined smectic branch to estimate the grand potential difference as μ is varied at fixed L, T, the absolute grand potential at one reference state point is obtained via the evaluation of the absolute Helmholtz free energy in the (N, L, A, T) canonical ensemble. It proceeds via a sequence of free energy difference estimations involving successively the cost of locali...

The relative stability and ergodicity of deterministic time-reversible thermostats, both singly and in coupled pairs, are assessed through their Lyapunov spectra. Five types of thermostat are coupled to one another through a single Hooke’s-law harmonic spring. The resulting dynamics shows that three specific thermostat types, Hoover–Holian, Ju–Bulgac, and Martyna–Klein–Tuckerman, have very similar Lyapunov spectra in their equilibrium four-dimensional phase spaces and when coupled in equilibrium or nonequilibrium pairs. All three of these oscillator-based thermostats are shown to be ergodic, with smooth analytic Gaussian distributions in their extended phase spaces (coordinate, momentum, and two control variables). Evidently these three ergodic and time-reversible thermostat types are particularly useful as statistical-mechanical thermometers and thermostats. Each of them generates Gibbs’ universal canonical distribution internally as well as for systems to which they are coupled. Thus they obey the zerot...

Molecular simulation could provide detailed description of fluid systems when compared to experimental techniques. They can also replace equations of state; however, molecular simulation usually costs considerable computational efforts. Several techniques have been developed to overcome such high computational costs. In this paper, two early rejection schemes, a conservative and a hybrid one, are introduced. In these two methods, undesired configurations generated by the Monte Carlo trials are rejected earlier than it would when using conventional algorithms. The methods are tested for structureless single-component Lennard–Jones particles in both canonical and NVT-Gibbs ensembles. The computational time reduction for both ensembles is observed at a wide range of thermodynamic conditions. Results show that computational time savings are directly proportional to the rejection rate of Monte Carlo trials. The proposed conservative scheme has shown to be successful in saving up to 40% of the computational tim...

The melting point of liquid lithium near zero pressure is studied with large-scale orbital-free first-principles molecular dynamics (OF-FPMD) in the isobaric-isothermal ensemble. We adopt the Wang-Govind-Carter (WGC) functional as our kinetic energy density functional (KEDF) and construct a bulk-derived local pseudopotential (BLPS) for Li. Our simulations employ both the ‘heat-until-melts’ method and the coexistence method. We predict 465 K as an upper bound of the melting point of Li from the ‘heat-until-melts’ method, while we predict 434 K as the melting point of Li from the coexistence method. These values compare well with an experimental melting point of 453 K at zero pressure. Furthermore, we calculate a few important properties of liquid Li including the diffusion coefficients, pair distribution functions, static structure factors, and compressibilities of Li at 470 K and 725 K in the canonical ensemble. Our theoretically-obtained results show good agreement with known experimental results, sugges...

Using the framework of Wertheim's thermodynamic perturbation theory, a new polyatomic density functional theory is developed to account for the intermolecular association of cyclic molecules in interfacial systems. To test the theory, Monte Carlo simulations in the canonical ensemble were performed for the specific case of an associating triatomic ring with one association site next to a hard wall. The theory and simulation results were found to be in good agreement.

We present an application of kinetic Monte Carlo (kMC) in the canonical ensemble to a calculation of vapour liquid equilibrium and to describe the adsorption of argon on a flat graphite surface and in a slit-like graphitic pore. Simulations at 77 and 87.3 K accurately describe the experimental data. The kMC method is simple to implement and, unlike conventional Monte Carlo, no rejection trials are necessary. The only move is a uniform sampling of the volume space, which makes the determination of the chemical potential straightforward using real particles in the simulation, in the same spirit as the Widom inverse potential distribution. This avoids the need to freeze the real particles before the trial insertion of test particles as is necessary in other methods, such as the Widom method and its variants.

A generalized methodology for statistical analogues of derivatives of the fundamental equation of state in the microcanonical and canonical ensembles is suggested. Those analogues include rigorous measuring prescriptions for any thermodynamic property in molecular NVE and NVT simulation. In particular, optimal data sets for exhaustive thermodynamic data correlation become rigorously accessible.

We study the influence of an external field on demixing and its interplay with the field-induced population inversion for a binary mixture of neutral and dipolar non-additive hard-spheres confined in a slit pore. Demixing lines are determined by Gibbs Ensemble Monte Carlo simulations and population inversion paths by grand-canonical/canonical simulations of the pore/bulk equilibrium. Besides the demixing line in the bulk, results are given for two different pore widths and in parallel and normal fields. Similar to the effect of geometrical confinement, a normal field is found to favour the mixed state so that the population inversion does not interfere with demixing. A parallel field leads to more complex scenarios.

The effect of ionic size on the diffuse layer characteristics of a spherical double layer is studied using Monte Carlo simulation and density functional theory within the restricted primitive model. The macroion is modelled as an impenetrable charged hard sphere carrying a uniform surface charge density, surrounded by the small ions represented as charged hard spheres and the solvent is taken as a dielectric continuum. The density functional theory uses a partially perturbative scheme, where the hard sphere contribution to the one particle correlation function is evaluated using weighted density approximation and the ionic interactions are calculated using a second-order functional Taylor expansion with respect to a bulk electrolyte. The Monte Carlo simulations have been performed in the canonical ensemble. The detailed comparison is made in terms of zeta potentials for a wide range of physical conditions including different ionic diameters. The zeta potentials show a maximum or a minimum with respect to ...

A systematic algorithm to design multiple thermostat systems in the framework of the Nose–Hoover type non-Hamiltonian formulation is presented. Using ‘non uniform’ time transformations in a generalised Hamiltonian equation, we develop the non-Hamiltonian equations of motion for multiple thermostat systems having an arbitrary number of thermostats and arbitrary connections between a physical system and thermostats (‘Nose–Hoover network’). We then present the algorithm to construct the Nose–Hoover network equations based on a simple diagram only. On the basis of this algorithm, recursively attached Nose–Hoover thermostats are introduced as an example of the Nose–Hoover network and its high efficiency in sampling the canonical distribution for an one-dimensional double-well system is illustrated by numerical calculations.

Using a combination of hybrid Monte Carlo simulations in the isothermal–isobaric ensemble with a Wang–Landau sampling, we parametrise a force field for polyaromatic hydrocarbons (PAHs). The proposed force field gives an accurate description of the vapour–liquid equilibria of naphthalene, phenanthrene and anthracene. In particular, the model yields a better account of the dependence of the vapour pressure on temperature than existing models. The strategy adopted in this work is markedly different from that followed in previous work, which relied on Gibbs Ensemble Monte Carlo (GEMC) simulations combined with configurational bias (CB) moves. The accuracy of the GEMC-CB method hinges on a reasonably high acceptance rate of the transfer of molecules from one phase to another, a condition difficult to achieve for large molecules like PAHs. Our approach avoids these transfers, allows for a direct determination of the coexistence data in the isothermal–isobaric ensemble and provides a promising alternative to the...

The thermodynamic properties of the triangular-well fluid with a well range of up to twice the hard sphere diameter were studied by means of a new developed equation of state and molecular simulation. This EoS is based on the perturbation theory of Barker and Henderson with the first and second-order perturbation terms evaluated by molecular simulation and then a fit with a simple function based on the radial distribution function of the reference fluid. The thermodynamic properties for the triangular-well fluid were also obtained directly by Gibbs ensemble and NPT Monte Carlo simulations. Good agreement is observed between the proposed EoS and the molecular simulation results. A model for the triangular-well solid is also presented; this has been used to calculate the solid–liquid transition line. Very good agreement is obtained with previously report values for this line and for the triple point temperature and pressure.

The canonical ensemble Monte Carlo method is applied to study the structure of polymer solutions confined between surfaces. The polymer molecules are modeled as fused-sphere freely rotating chains with fixed bond length and bond angles and the solvent as hard spheres. The simulation results for the configurational and conformational properties of the chains are presented with varying interfacial distances, chain concentrations, and chain lengths. The chains are depleted at the wall at lower density, which, however, becomes less at higher density. With an increase in the interfacial distance, the enhancement/depletion of the chains at the wall becomes more marked. At all interfacial distances and chain lengths, increasing the concentration of the solvent makes the oscillation in the density profile of the chains more pronounced. Conformational properties provide important indications regarding the behaviour of chains as they approach surfaces.

The molecular dipole moments and local environments of water in its liquid phase were examined for a series of first-principles Gibbs ensemble Monte Carlo simulations along the vapour–liquid coexistence curve using the Becke–Lee–Yang–Parr (BLYP) and Perdew–Burke–Ernzerhof (PBE) exchange/correlation density functionals. Molecular dipole moments were computed using maximally localized Wannier functions with the Berry phase scheme, while the structure was analysed with respect to tetrahedral order parameter and hydrogen bonding. Increasing the temperature results in a decrease of both the average molecular dipole moment and the local structure, although the width of the dipole distribution remains fairly constant. A correlation is found between the extent of the local structure and the magnitude of the molecular dipole moment, but this correlation is limited to the first solvation shell.

Gibbs ensemble Monte Carlo simulations were run with an efficient mixed-basis electronic structure method to explore the phase equilibria of water from first principles using Kohn–Sham density functional theory. The Perdew–Burke–Ernzerhof exchange/correlation density functional gives a higher critical temperature (700 K) and boiling point (480 K) than experiment, although good agreement is found for the saturated liquid densities. A systematic increase in the size of the basis set for the Becke–Lee–Yang–Parr exchange/correlation density functional from a double-ζ to quadruple-ζ split valence leads to further deviations from experiment on the saturated liquid and vapor densities, while the intermediate basis set gives the best results for the heat of vaporization at T = 423 K. Analysis of the liquid structure for all simulations shows changes that can partially be explained by the different densities at a given temperature, and both density functionals show a similar temperature dependence of the liquid st...

We present new molecular dynamics results for the pressure of the pure hard disk fluid up to the hexatic transition (about reduced density 0.9). The data combined with the known virial coefficients (up to $B_{10}$) are used to build an equation of state, to estimate higher-order virial coefficients, and also to obtain a better value of $B_{10}$. Finite size effects are discussed in detail. The ``van der Waals-like'' loop reported in literature in the vicinity of the fluid/hexatic transition is explained by suppressed density fluctuations in the canonical ensemble. The inflection point on the pressure-density dependence is predicted by the equation of state even if the hexatic phase simulation data are not considered., 9 pages, 3 figures, presented at The Seventh Liblice Conference on the Statistical Mechanics of Liquids (Lednice, Czech Republic, June 11--16, 2006)

We consider some fundamental aspects of the calculation of the pressure from simulations of systems characterized by discontinuous interactions. The usual approach to the computation of the pressure in such systems is based on the mechanical (or virial) expression, which in turn can be expressed in terms of the value of the radial distribution function at contact or the number of overlaps that would occur under a virtual compression of the system. A less widespread, and formally equivalent, approach invokes the thermodynamic definition of the pressure and involves the estimate of the change in free energy associated with a small volume perturbation. There are, however, important differences in the way the virial and thermodynamic evaluations of the pressure are implemented, and, as a consequence, in the corresponding errors associated with each technique. The bulk pressure is determined with the virial and the thermodynamic routes for the hard-sphere system in the fluid and solid states. Good agreement is...

The structure and phase-equilibrium properties of CO2-expanded acetonitrile (MeCN) were examined using Gibbs Ensemble Monte Carlo simulations. The results showed that the mole fraction of the CO2 in the binary system increased linearly with pressure and the predicted volume expansion of the liquid phase with pressure is nonlinear and in agreement with experiment. The site–site radial distribution functions (RDFs) were determined for this binary mixture, which show that the homomolecular CO2–CO2 and heteromolecular CO2–MeCN do not exhibit a significant dependence on CO2 mole fraction. The only major structural change that is seen to occur as the CO2 mole fraction is increased in the mixture is a decrease in the fraction of nearest neighbour MeCN molecules with parallel dipole orientations.

The SAFT-VR equation of state is extended to treat heteronuclear chain fluids, focusing, in particular, on symmetric and asymmetric diblock chains. The chain molecules studied are composed of segments of different size and/or energy of interaction. Both symmetric and asymmetric systems are considered. The theoretical predictions are compared with isothermal–isobaric and Gibbs ensemble Monte Carlo simulation data. Excellent agreement is obtained between the hetero-SAFT-VR predictions and the simulation data, validating the use of the SAFT-VR approach for heteronuclear chains in more realistic models of polymers and small molecules composed of different functional groups.

Simulation results for a system comprising a square well plus either a point dipole or a point quadrupole are presented. The properties obtained are the vapour–liquid equilibrium densities and the critical properties. Critical densities are not very sensitive to the values of dipole or quadrupole, while critical temperatures increase significantly when the multipole strength rises. A comparison with a perturbation theory for multipolar square-well systems is presented. Overall agreement between simulated and theoretical values is good when comparison is restricted to quadrupoles or dipoles corresponding to the most relevant real polar substances but is only moderate for the largest multipolar strengths considered.

The Maxwell–Boltzmann distribution of molecular velocities and densities was derived first in the 1870s, but the derivations were difficult to follow and led to predictions of the heat capacities of gases that did not agree with experiment. The systems discussed were molecules or assemblies of molecules to which a definable energy could be ascribed, the equivalent of the microcanonical ensemble of Gibbs. This led to complications when dense gases and liquids were considered, and so, for the next thirty years, to arguments about the validity of the derivations. In principle these complications can be removed most simply by considering a set of systems of fixed temperature, or a canonical ensemble. It was, however, many years before most textbooks followed Gibbs down this route.

Using a newly developed algorithm in the Gibbs ensemble, the liquid–vapour phase diagram of methane adsorbed in single and multilayer nanotubes was considered. The method was based on improving statistical sampling by combining the cavity-bias and the parallel tempering formalisms in Gibbs ensemble Monte Carlo simulations (PTCBGMC). Two models were constructed in order to describe the liquid–vapour equilibrium of methane in nanotubes. The first model consisted of two simulated nanotube-shaped boxes, one containing the vapour and the other containing the liquid. The vapour and liquid were kept in thermodynamic equilibrium using the PTCBGMC algorithm. Cailletet–Mathias phase diagrams showed that upon adsorption of methane in the nanotubes, the critical temperature and density of methane decreased upon confinement. However, the behaviour of the liquid and vapour phases differed depending on whether the nanotube was single- or multi-layered. A second computational model was used to consider in detail the capi...

An equation of state for square-well fluids of short and long potential range λ is presented and compared with Gibbs ensemble and canonical Monte Carlo simulation data; vapour–liquid coexistence densities, vapour pressures, internal energies and contact radial distribution functions are examined. The equation is an extension of that presented in previous work for the reference monomer fluid in the SAFT-VR approach [GIL-VILLEGAS et al., 1997, J. chem. Phys., 106, 4168]. The Helmholtz free energy is written as a high-temperature expansion up to second order, where simple expressions are obtained for the mean attractive energy and the fluctuation term using the mean-value theorem and a mapping of radial distribution functions. In previous work the range of the square-well potential was limited to λ ≤ 1.8. In this work we show that the phase behaviour of such a fluid is far from the expected long-range limits given by the mean-field and van der Waals approximations. We extend the applicability of the equation...

Self-adapting fixed endpoint configurational-bias Monte Carlo simulations in the Gibbs ensemble were carried out to determine the vapour–liquid coexistence curves of cyclic alkanes from c-pentane to c-octadecane. In general, the critical temperatures and densities of the cyclic alkanes are substantially higher than those of their linear counterparts. Furthermore, the simulation data point to a maximum in the critical density for cyclic alkanes with about eight carbon atoms as also observed for the linear alkanes.

In this work the Gibbs–Duhem integration method is used in combination with molecular simulation in the semigrand ensemble to simulate vapour–liquid equilibria for the binary mixtures ethane–methane and carbon dioxide–dimethylsulphoxide (DMSO). In both binary mixtures, the two components have a large difference in relative volatility. The starting point for tracing the vapour–liquid phase envelope is either calculated in a pure species NVT Gibbs ensemble simulation or in a pure species NpT+test particle simulation. The chosen method depends on the vapour pressure of the pure species. An equation is presented that relates the initial slope of the Clapeyron equation to an ensemble average in the Gibbs ensemble. The NpT+test particle method is slightly modified in order to improve the convergence characteristics when one of the coexisting phases is a (very) dense phase. Configurational-bias methods are used in the NVT Gibbs ensemble to enhance the acceptance of particle exchanges, and in the NpT+test particl...

Chiral liquid crystals have been investigated by means of a multicanonical Monte Carlo approach in order to characterize their phase behaviour by microcanonical equilibrium properties. The liquid crystals were described by three-dimensional lattice systems with intermolecular interactions given by the chiral Lebwohl-Lasher potential. Self-determined boundary conditions have been applied in order to enable the formation of chiral phases with equilibrium pitch. Selected thermodynamic properties, e.g. microcanonical entropy, temperature, heat capacity and a set of order parameters have been determined with dependence on microcanonical total energy. A cholesteric phase with temperature-induced helix inversion could be proven where the helical superstructure of the single component system studied changed its handedness through an infinite-pitch system. The thermodynamical behaviour in the microcanonical ensemble was found to be very similar to the behaviour in the canonical ensemble. The study of microcanonica...

A Gibbs ensemble Monte Carlo (GEMC) study supplemented by theoretical calculations using the mean spherical approximation (MSA) is reported for the charged Yukawa system. In this system the particles interact via an attractive Yukawa potential and a Coulomb potential. When the Coulomb potential is weak compared to the Yukawa attraction the results obtained for the vapour-liquid equilibrium from the GEMC and the MSA are very similar. On increasing the charge, the system becomes similar to the primitive model of electrolytes, and at high charges similar phenomena, such as ion association, are observed. At these charges, the MSA yields poorer results. Using both methods, an increase of the charge results in an increase of the critical temperature but a decrease of the critical density.

Molecular dynamics simulations of Lennard-Jones binary mixtures were performed to obtain phase equilibria and thermodynamic properties for the liquid—vapour interface. The dispersion interactions were handled using the lattice sum method where the full interaction is obtained and there is no requirement for any long range correction to the properties. The application of the method using the Lorentz—Berthelot combining rule for unlike interactions is discussed. The coexisting densities, adsorption of molecules at the interface and surface tension are the main results of this work. Coexisting properties were compared with Gibbs ensemble Monte Carlo results and with those of the grand canonical Monte Carlo method combined with the histogram reweighting technique, and good agreement was found. The lattice sum method results were compared with those of the spherically truncated and shifted potential to analyse the truncation effect. The adsorption of molecules at the interface and surface tension increase with...

A powerful extrapolation scheme is proposed to determine the vapour—liquid and liquid—liquid equilibrium curves of mixtures by performing a single isothermal-isobaric Gibbs ensemble Monte Carlo (GEMC) simulation. The coexistence curves for the mole fraction and the density are extrapolated as functions of the temperature and the pressure by second-order Taylor series. The coefficients of the Taylor series, which are the temperature and pressure derivatives of these quantities along the coexistence curves, can be calculated from the data produced by a single GEMC simulation on the basis of fluctuation formulas. We show that the application of a Pade approximant considerably widens the temperature and pressure range where the extrapolation is accurate. Using Lennard-Jones mixtures as test systems, we show that the technique is able to produce quite accurate equilibrium curves at fixed temperature in the function of the pressure and vice versa. The procedure yields good results not only for vapour—liquid but...

Molecular dynamics simulations in the canonical ensemble have been performed to obtain the thermodynamic and transport properties of the Lennard-Jones fluid. The dispersion interactions were calculated using lattice sums. This method makes it possible to simulate the full potential avoiding the inclusion of the long range corrections (LRC) during or at the end of simulations. In the calculation of dynamic properties in bulk phases and thermodynamic quantities of inhomogeneous systems where the interface is physically present, in general the LRC cannot easily be included. By using the lattice sums method, the results are independent of the truncation of the potential. In the liquid-vapour interface simulations it is not necessary to make any pre-judgments about the form of the LRC formula to calculate coexisting properties such as the surface tension. The lattice sums method has been applied to evaluate how well the full interaction can be calculated in the liquid phase and in the liquid-vapour interface. ...

The equilibrium between vapour and liquid in a square-well system has been determined by a hybrid simulation approach combining chemical potentials calculated via the Gibbs ensemble Monte Carlo technique with pressures calculated by the standard NVT Monte Carlo method. The phase equilibrium was determined from the thermodynamic conditions of equality of pressure and chemical potential between the two phases. The results of this hybrid approach were tested by independent NPT and μPT calculations and are shown to be of much higher accuracy than those of conventional GEMC simulations. The coexistence curves, vapour pressures and critical points were determined for SW systems of interaction ranges λ = 1.25, 1.5, 1.75 and 2. The new results show a systematic dependence on the range λ, in agreement with results from perturbation theory where previous work had shown more erratic behaviour.

Ab initio pair interaction energies for methyl fluoride (CH3F) were calculated using symmetry adapted perturbation theory with the aug-cc-pVDZ basis set plus bond functions for a large number of configurations of a pair of methyl fluoride molecules in order to obtain a good representation of the two-body potential energy surface. These interaction energies were used to develop a site-site intermolecular potential that accurately reproduced the calculated energies and had the correct asymptotic behaviour at long range. This pair potential was used in Gibbs ensemble Monte Carlo simulations to predict the phase behaviour of methyl fluoride. The predicted equilibrium properties with the ab initio pair potential are in good agreement with the experimentally measured phase boundary, and give an estimated critical point in excellent agreement with measured values. Multi-body effects were then added to the pair potential using a polarizable model. The small changes to the phase behaviour that resulted established...

A powerful extrapolation scheme is proposed to determine the vapour±liquid equilibrium (VLE) curves of pure molecularuids over a ®nite temperature range by performing a single Gibbs ensemble Monte Carlo (GEMC) simulation. The VLE curves of the various thermo- dynamic quantities are extrapolated as functions of the temperature by second-order Taylor series. The coeA cients of the Taylor series, which are the temperature derivatives of these quantities along the VLE curves, can be calculated from a single GEMC simulation using Clapeyron-like equations anductuation formulas containing double or triple correlations. It is shown that the application of a Pade approximant considerably widens the temperature range where the extrapolation is accurate. The eA ciency of the method is demonstrated by applying it on the Lennard-Jonesuid and a three-site model of carbon dioxide. The pro- cedure is found to be a very useful tool that is able to reproduce the VLE curves over a considerable temperature range. The procedure makes it possible to calculate the saturation heat capacity fromuctuation formulas.

A first step towards the development of a general, realistic potential model for perfluoroether compounds has been to parameterize a united atom model for a short chain perfluoroether perfluoromethylpropyl ether (CF3CF2CF2OCF3). The potential model takes the usual form in which separate bond bending and torsional terms describe the intramolecular interactions with the addition of van der Waals and electrostatic terms to describe the non-bonded interactions. Ab initio quantum calculations have been carried out to obtain the partial charges and intramolecular torsional and bending potentials. Phase equilibrium data were then used to optimize the van der Waals interaction parameters through Gibbs ensemble Monte Carlo simulations. The resulting model reproduces vapour-liquid equilibrium densities, the critical temperature and the critical density of perfluoromethylpropyl ether, in good agreement with those from experiment.

A molecular dynamics simulation method which can generate configurations belonging to the canonical (T, V, N) ensemble or the constant temperature constant pressure (T, P, N) ensemble, is proposed. The physical system of interest consists of N particles (f degrees of freedom), to which an external, macroscopic variable and its conjugate momentum are added. This device allows the total energy of the physical system to fluctuate. The equilibrium distribution of the energy coincides with the canonical distribution both in momentum and in coordinate space. The method is tested for an atomic fluid (Ar) and works well.