Configurational-bias grand-canonical transition-matrix Monte Carlo simulations are conducted to investigate various thermophysical properties, such as phase coexistence, critical properties, density and orientation profiles of liquid and vapor phases, and vapor-liquid surface tension of methane, ethane, propane, n-butane, and n-octane in bulk and slit pores of graphite and mica surfaces. An exponential-6 (exp-6) model is used for the investigation of normal alkanes with a cutoff radius, 15 Å. It is found that the surface tension of the bulk n-alkane, C 1 -C 4 , based on a truncated exp-6 model, agrees reasonably well with the experimental data. Critical properties are reported by means of the rectilinear diameter approach and least-squares technique. The shift in the critical temperature under confinements follows more than two linear regimes with an inverse in the slit width, as the slit width approaches the two-dimensional limit. This is contrary to what has been previously reported in the literature. The behavior of the critical temperature shift is sensitive to the nature of the surface. The critical density, on the other hand, fluctuates with a decrease in the slit width. The shift in the critical vapor pressure continuously increases with a decrease in the slit width toward the two-dimensional value and becomes constant for pore sizes less than 5 Å for the fluid studied in this work. Corresponding state plots suggest that the deviation of the saturation vapor pressure from the bulk saturation pressure under confinement is positive for large pores and negative for smaller pores. Vapor-liquid surface tension values for n-alkanes, in both types of slit pores, are computed via the finite-size scaling method of Binder and compared with their bulk values. Our investigation reveals that under slit-pore confinement the vapor-liquid surface tension decreases many fold.

We consider the accuracy of several methods for combining forward and reverse free-energy perturbation averages for two systems ͑labeled 0 and 1͒. The practice of direct averaging of these measurements is argued as not reliable. Instead, methods are considered of the form ␤(A 1 ϪA 0) ϭϪln͓͗w(u)exp(Ϫ␤u/2)͘ 0 /͗w(u)exp(ϩ␤u/2)͘ 1 ͔, where A is the free energy, ␤ϭ1/kT is the reciprocal temperature, uϭU 1 ϪU 0 is the difference in configurational energy, w(u) is a weighting function, and the angle brackets indicate an ensemble average performed on the system indicated by the subscript. Choices are considered in which w(u)ϭ1 and 1/cosh͓(uϪC)/2͔; the latter being Bennett's method where C is a parameter that can be selected arbitrarily, and may be used to optimize the precision of the calculation. We examine the methods in several applications: calculation of the pressure of a square-well fluid by perturbing the volume, the chemical potential of a high-density Lennard-Jones system, and the chemical potential of a model for water. We find that the approaches based on Bennett's method weighting are very effective at ensuring an accurate result ͑one in which the systematic error arising from inadequate sampling is less than the estimated confidence limits͒, and that even the selection w(u)ϭ1 offers marked improvement over comparable methods. We suggest that Bennett's method is underappreciated, and the benefits it offers for improved precision and ͑especially͒ accuracy are substantial, and therefore it should be more widely used.

Vapor-liquid interfacial tension of square-well ͑SW͒ fluids is calculated using three different methods viz., molecular dynamics ͑MD͒ with collision-based virial evaluation, Monte Carlo with virial computed by volume perturbation, and Binder's density-distribution method in conjunction with grand-canonical transition-matrix Monte Carlo ͑GC-TMMC͒. Three values of the SW attractive well range parameter were studied: ϭ1.5, 1.75, and 2.0, respectively. The results from MD and GC-TMMC methods are in very good mutual agreement, while the volume-perturbation method yields data of unacceptable quality. The results are compared with predictions from the statistical associating fluid theory ͑SAFT͒, and SAFT is shown to give a good estimate for the systems studied. Liquid and vapor coexistence densities and saturation pressure are determined from analysis of GC-TMMC data and the results are found to agree very well with the established literature data.

We use the Mayer sampling method, with both direct and overlap sampling, to calculate and compare classical virial coefficients up to B6 for various water models (SPC, SPC/E, MSPC/E, TIP3P, and TIP4P). The precision of the computed values ranges from 0.1% for B2 to an average of 25% for B6. When expressed in a form scaled by the critical properties, the values of the coefficients for SPC water are observed to greatly exceed the magnitude of corresponding coefficients for the simple Lennard-Jones model. We examine the coefficients in the context of the equation of state and the Joule-Thomson coefficient. Comparisons of these properties are made both to established molecular simulation data for each respective model and to real water. For all models, the virial series up to B5 describes the equation of state along the saturated vapor line better than the series that includes B6. At supercritical temperatures, however, the sixth-order series often describes pressure-volume-temperature behavior better than the fifth-order series. For example, the sixth-order virial equation of state for SPC/E water predicts the 673 K isotherm within 8% of published molecular simulation values up to a density of 9 mol/L (roughly half the critical density of SPC/E water).