Divalent oxygen with one hydrogen attached Our original goal was to refit all LJ SMIRKS types, but there were insufficiently diverse physical property training data (determined as having fewer than 5 data points for either density or enthalpy of mixing) for some chemistries. This refit covered most parameters for the chemical space of hydrogen, carbon, nitrogen, oxygen, chlorine, and bromine; notable types that were not refit describe fluorine, phosphorous, and sulfur. The training data, described in detail in Section 2.2.2, consisted of measurements of densities of pure systems 𝜌, densities of binary mixtures 𝜌 𝑚𝑖𝑥 , and enthalpies of mixing of binary mixtures Δ𝐻 𝑚𝑖𝑥 . Optimization was performed by iteratively minimizing a ForceBalance objective function 𝐿 𝐿𝐽 (𝜃), a weighted least squares objective comparing simulation estimates of training data points with their experimental values, shown in Equation .