where N is the number of sites on an atom, r ij is the distance from the site to the sampling point and q j is the charge on site j. An additional threshold parameter (F thresh ) was required to distinguish between atoms that required extra sites and those that did not. Above this threshold the anisotropy method is used, below the threshold, no off-center charges are added. As well as this, extra charges are only added when there is a reduction in error which is controlled by a second parameter (F change ).