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.