NPT Test Particle Method

The NPT + test particle method [8, 9] aims to determine phase coexistence points based on calculations of the chemical potentials for a number of state points. A phase coexistence point is determined at the intersection of the vapor and liquid branches of the chemical potential versus pressure diagram. The Widom test particle method [7] of the previous paragraph or any other suitable method [10] can be used to obtain the chemical potentials. Corrections to the chemical potential of the liquid and vapor phases can be made, using standard thermodynamic relationships, for deviations 

In contrast to the Gibbs ensemble discussed later in this chapter, a number of simulations are required per coexistence point, but the number can be quite small, especially for vapor-liquid equilibrium calculations away from the critical point. For example, for a one-component system near the triple point, the density of the dense liquid can be obtained from a single NPT simulation at zero pressure. The chemical potential of the liquid, in turn, determines the density of the (near-ideal) vapor phase so that only one simulation is required. The method has been extended to mixtures [12, 13]. Significantly lower statistical uncertainties were obtained in [13] compared to earlier Gibbs ensemble calculations of the same Lennard-Jones binary mixtures, but the NPT + test particle method calculations were based on longer simulations. 

---

III. THE NPT+ TEST PARTICLE METHOD, GIBBS-DUHEM INTEGRATION AND PSEUDO-ENSEMBLES... 

The NPT + test particle method [33,34] is based on calculations of the ehemieal potentials for a number of state points. A phase coexistence point... 

Interesting connections between many of the methods discussed in the present chapter have been pointed out by Escobedo [54,55]. In particular, Escobedo suggests that Gibbs-Duhem integration, pseudo-ensembles, and the NPT + test particle method can be considered as low-order approximations of a histogram reweighting approach. 

Moller, D. Fischer, J., Vapour liquid equilibrium of a pure fluid from test particle method in combination with npt molecular dynamics simulations, Mol. Phys. 1990, 69, 463 173... 

Fotfi, A. Vrabec, J. Fischer, J., Vapour liquid equilibria of the Lennard-Jones fluid from the NPT plus test particle method, Mol. Phys. 1992, 76, 1319-1333... 

Boda, D. Fiszi, J. Szalai, I., An extension of the NPT plus test particle method for the determination of the vapour-liquid equilibria of pure fluids, Chem. Phys. Lett. 1995, 235, 140-145... 

Vrabec J, Hasse H (1995) Vapour liquid equilibria of mixtures from the NpT plus test particle method. Mol Phys 85 781-792... 

---