We note that the method of Chmiela et al. for generating potential energy surfaces of small organic molecules 65 uses the same kernel ridge regression technique with the following differences: (i) they include only gradient observables (i.e. forces) while GAP reconstructs the surface using both potential energies and forces, (ii) they use the same number of basis functions as there are data, which corresponds to M = 3N n above, (iii) their basis functions for the potential energy are derivatives of a base kernel (such as a Gaussian) with respect to atomic positions, rather than the base kernel itself, (iv) they use the inverse of interatomic distances as the arguments of the kernel. We have found no significant advantage to any of these variations, and note that (ii) would result in a larger linear problem, thus significantly increasing the computational cost. We typically find that M 3N n is sufficient.