The goal of the GAP training procedure was to recreate the QM potential energy surface of 3BPA at the MP2/6-311G(2d,p) level of theory. This choice represents a compromise between accuracy and computational expense; one energy and force evaluation requires approximately 1 cpuhr, which makes it feasible to generate thousands of data points. However, accurate characterization of the multidimensional potential energy surface requires extensive sampling, which is not practical with an expensive QM method, so we used the following protocol.