We now briefly outline the Gaussian Approximation Potential (GAP) 46 framework, and how we apply it to create a potential energy surface for 3BPA that reproduces quantum mechanical energies to within 1.0 kcal mol -1 root mean square (RMS) error. GAP has been applied to many different materials and compounds, and has been described in detail elsewhere, and so we summarize here only the main features. Although the probabilistic and linear regression viewpoints are entirely equivalent, we follow the latter here because it is likely to be more familiar, and we will not be making use of the uncertainty estimates and parameter optimization techniques that follow naturally from the former.