(3) i . The angular distribution function is also expended in terms of spherical harmonics, which are the angular basis functions. The expansion coefficients of the radial and angular neighbour densities are collected into a vector to form the descriptor of the configuration of atom i as X i . A Gaussian Approximation Potential (GAP) is used to relate the descriptors to the atomic potential energy U i . The potential is defined by a set of N B training configurations {X iB | i B = 1, ..., N B } and fitting parameters w iB , with similarity between two configurations measured us-ing a polynomial kernel K: