The diagonal elements in Λ were set to be 10 6 , and the length scale parameter θ i was chosen to be 20 times the range of the data distribution in each dimension of the configuration vector R. The RMS errors of the fitted potential on the 2000 test configurations were 0.57 kcal mol -1 for the energy and 0.95 kcal mol -1 Å-1 for the forces.