Defining the generalised work function ϕ = βU +ln (q), one can use the two-state BAR equation (extensively discussed in Ref. ) to estimate absolute FE of each system. The ensemble averages over MD data (⟨•⟩ p ) used in the two-state BAR estimator was here evaluated on locally ergodic MD data, exclusively sampled from the validation dataset (i.e. data not included in the PGM training set). As such, we refer to this estimator as BAR V and indicate the absolute free energy estimates in reduced units obtained from this estimator as f BAR V crys , where the subscript crys indicates the polymorph. In each system, f BAR V crys was evaluated on random batches of 10 4 validation data points during model training, with a stride of 25 training batches between evaluations. This reweighing approach is both computationally cheap and accurate, an observation that aligns well with results in prior literature. Furthermore, the use of the pymbar library to evaluate the BAR free energy estimator provides analytical error bars which quantify the uncertainty of f BAR V crys estimates.