A more accurate version of the Transferable Potentials for Phase Equilibria-United Atom force field, called TraPPE-UA2, for ethane and ethylene is presented. Very similar molecular volumes, shapes, and self- and cross-interaction strengths of ethane and ethylene make their separation fundamentally interesting and industrially challenging. Separation factors as low as 1.5-3.0 necessitate very accurate molecular models in order to be able to computationally design potential separation processes. Additional force field parameters, namely the distance between the Lennard-Jones sites for both compounds and partial charges only for ethylene, are introduced in the parameterization and different combining rules for the Lennard-Jones interaction are considered. In addition to the liquid densities and critical temperature, the training set also includes saturation vapor pressures to yield an accurate two-site ethane model. Binary ethane/ethylene, CO2/ethylene, and H2O/ethylene vapor-liquid equilibria and H2O/ethylene dimer calculations are used for further optimization of a four-site ethylene model. (c) 2017 American Institute of Chemical Engineers