A new series of united atom force fi elds named optimized potentials for phase equilibria simulation (OPPES) are presented.

Many classical force fi elds 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 diff ers from the typical value

of 1.54 Å in previous united atom force fi elds. Some of the bonded interaction constants for CH 2 , CH 3 pseudo-atoms, and

ether oxygen were determined using the density functional theory calculations, while others were taken from the TraPPEUA,

NERD, OPLS-UA, and AMBER models. The interatomic parameters of united atom potentials for linear alkanes and

ethers were optimized by fi tting 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 fi eld

for phase equilibria simulation. The OPPES model showed signifi cant improvements for ethers while providing accurate

phase equilibria description for short-chain n-alkanes comparable to the TraPPE-UA model.