Advancing Force Field Accuracy: The Essential Role of Bond Length in Vapor–Liquid Equilibria Simulations for n-Alkanes and Ethers

A new series of united atom force fields named optimized potentials for phase equilibria simulation (OPPES) are presented. Many classical force fields have been proposed for predicting various physicochemical properties. The OPPES aims to improve the accuracy of direct-phase equilibrium simulations, particularly for vapor–liquid equilibria. The main feature of OPPES is the use of a new bond length of carbon pseudo-atoms involving methyl groups, which differs from the typical value of 1.54 Å in previous united atom force fields. Some of the bonded interaction constants for CH₂, CH₃ pseudo-atoms, and ether oxygen were determined using the density functional theory calculations, while others were taken from the TraPPE-UA, NERD, OPLS-UA, and AMBER models. The interatomic parameters of united atom potentials for linear alkanes and ethers were optimized by fitting to the selected properties such as vapor pressure and saturated liquid density, followed by a Gibbs ensemble Monte Carlo simulation to evaluate the performance of the newly determined potential parameters. The simulation results were compared to those obtained using the TraPPE-UA model, currently the best united atom force field for phase equilibria simulation. The OPPES model showed significant improvements for ethers while providing accurate phase equilibria description for short-chain n-alkanes comparable to the TraPPE-UA model.