'Peakiness' test functions

A number of common functions have been tested to determine their efficiency in refining structures using the criteria of 'peakiness' and non-negativity of the electron density distribution. It is concluded that the best function to maximize, generally, is ∫ ρ³dv, although, in special cases, a higher power of ρ may be better. When there is a heavy atom in the structure the method is much less sensitive to the particular function used so long as it involves powers of ρ greater than ρ².