Next, consider the large difference in evaluation speed of energies and gradients between the MB and F-PESs. While the MB-PES has a smaller number of linear coefficients than the F-PES, there are many more terms to evaluate in the former. By contrast, the evaluation of the F-PES is "once and done". All MB approaches are hampered by the number of terms to evaluate and, as noted, this number is given by as ( N n ) which goes as O(N n /n!). However, by applying a physically reasonable cutoff for each n-body term the actual number is far less.