Assessing the Quality of Molecular Simulations for Vapor–Liquid Equilibria: An Analysis of the TraPPE Database

Abstract

As molecular modeling and simulation techniques become increasingly important sources of thermophysical property and phase equilibrium data, the ability to assess the robustness of that data becomes more critical. Recently, the use of the compressibility factor (Z) has been suggested as a metric for testing the quality of simulation data for vapor− liquid equilibria (VLE). Here, we analyze predicted VLE data from the transferable potentials for phase equilibria (TraPPE) database and show that, apart from data entry or typographical errors, Z will always be well-behaved in Gibbs ensemble Monte Carlo (GEMC) simulations even when the simulations are not sufficiently equilibrated. However, this is not true for grand canonical Monte Carlo simulations. When the pressure is calculated from the internal forces, then pressure and density are strongly correlated for the vapor phase and, for GEMC simulations, it is recommended to treat Z as an instantaneous mechanical property. From analysis of the TraPPE VLE data, we propose a complementary metric based on the predicted vapor pressures at three neighboring temperatures and their deviation from a local Clausius−Clapeyron fit.

A Force Field for Poly(oxymethylene) Dimethyl Ethers (OMEn)

“…Obtaining vapour-liquid equilibria (VLE) with molecular simulations is challenging. Although simulations of VLE at such low pressures have been carried out previously, 25 either the chain length of the molecules was smaller or the liquid density was lower than is the case for the OMEn studied in the present work. As a result, in the present work, only one data point of the vapour pressure can be used in the fit for each OMEn with n ě 2.…”

Study on Lubrication Characteristics of C4-Alkane and Nanoparticle during Boundary Friction by Molecular Dynamics Simulation

“…The C4-alkane is simulated by the TraPPE United Atom force field which utilizes pseudo-atoms to represent the CHx groups [43]. It should be noted that, TraPPE United Atom for field is not the best one to accurately describe the material properties compared to other advanced force fields, but it is still widely used from the consideration of computational efficiency [25][26][27][43][44][45]. Thus, we have applied the TraPPE United Atom force filed for atomistic simulations in order to reduce the computational cost, which is very important for NEMD simulations when there are a huge number of cases to run.…”

Partial molar properties from molecular simulation using multiple linear regression

“…8) can be computed directly using the instantaneous enthalpy of a given simulation frame H = U + pV , from instantaneous measures of U and V . A comparison of the instantaneous and ensemble average methods is discussed in Section 4.1.2, and also in recent work by Eggimann et al [43].…”

“…For finite-sized systems, correlations may influence the ensemble averages of derived properties such as the compressibility factor, Z [43], or in this case, H. To explore this closely, we compute H andH i two ways.H i,inst are computed using Eq. 8, after computing the mechanical observable H inst using instantaneous measures of U and V for each frame, and p is taken as either the instantaneous value for N V T -Gibbs simulation or as the specified thermodynamic constraint for N pT -Gibbs simulations.…”