In this equation, ,f f is the simulation estimate of with a given force field, and is the experimental value of . The reference force field f f0 is always chosen as OpenFF 1.0.0 in this analysis. The average is taken over the test set of physical properties of one specific type (e.g. over the 236 Δ ( ) measurements in our test set). When bootstrapped 95% confidence intervals are calculated with this metric, bootstrapping is performed over paired measurements in two force fields, capturing the correlation between force fields that is lost when bootstrapped errors are calculated individually. We also calculate kernel density estimates (KDE) of the distribution of individual shifts