The parameters of the anisotropic united atoms potential for linear alkanes proposed by Toxvaerd [S. Toxvaerd, J. Chem. Phys. 107, 5197 (1997)] have been optimized on the basis of selected equilibrium properties (vapor pressures, vaporization enthalpies, and liquid densities) of ethane, n-pentane, and n-dodecane. The optimized parameters for the CH2 and CH3 groups form a regular sequence with those of methane and the force centers are found between the carbon and hydrogen atoms, as expected. The resulting potential, called AUA4, has been compared with Toxvaerd’s potential (AUA3) by using several molecular simulation methods (Gibbs ensemble Monte Carlo, thermodynamic integration, and molecular dynamics). An investigation performed at temperatures ranging from 140 to 700 K and with various chain lengths up to 20 carbon atoms has shown AUA4 to provide systematic improvements of vapor pressures, vaporization enthalpies, and liquid densities for pure n-alkanes. Significant improvements have been also noticed on the critical temperatures of n-alkanes, estimated from coexistence density curves, and on the equilibrium properties of CO2–n-alkane binary mixtures. Self-diffusion coefficients of n-hexane, however, are slightly improved by the new potential, but still exceed experimental measurements at low temperature. As we have only optimized the intermolecular potential in the present study, it is suggested that further optimization of the intramolecular potentials of the anisotropic united atoms model could allow simultaneous prediction of thermodynamic properties and of transport coefficients, particularly in very dense liquids.

A Monte Carlo replica exchange (MCRE) algorithm was used to compute the sodium cation distribution in bare faujasite zeolite with a number of cations per unit cell ranging from zero (Si:Al → ∞) to 96 (Si:Al = 1). Eight independent realizations of the system were simulated simultaneously, in the temperature range 300-2325 K. The resulting distributions at room temperature were found to be in very good agreement with both available experiments and the analytical quasi-chemical model of Mortier and co-workers. A single canonical simulation at room temperature using site-to-site hopping yields identical results. The main advantage of the MCRE method is that no assumption is needed on the actual cation adsorption sites. One could thus, in principle, predict cation locations and distributions in a nanoporous solid for which the precise location of extraframework cations is not known.