where N conf is the number of configurations in training set, N atom is the number of atoms in each snapshot, ⃗ µ P IM are the dipole moments obtained from PIM and ⃗ µ DF T are the reference dipole Finally, the parameters of the short-range repulsion term (V rep ) and b IJ disp in V disp are optimized following a similar procedure by fitting the forces calculated from PIM to those obtained from DFT: