The "pure only" optimization task optimizes the LJ parameters against 56 physical properties (𝜌 𝐿 , Δ𝐻 𝑣𝑎𝑝 ) for a set of 28 molecules, which are shown in Figure . A list of the molecules in the "pure only" training set are available in the Supporting Information, Section S1.1. The measurements here are either sourced from the NIST ThermoML Archive (𝜌 𝐿 ) or handcurated from literature (Δ𝐻 𝑣𝑎𝑝 ) , because of the low number of Δ𝐻 𝑣𝑎𝑝 data points in the ThermoML Archive. All measurements are selected at temperatures and pressures close to ambient (∼1 atm, 273.15-318.15 K).