where R A denotes the location of an atom, and γ i are decay lengths (γ 1 = 0.6 a.u., γ 2 = 0.9 a.u. and γ 3 = 1.35 a.u.). Once the model density is fit, the charges on the atoms are calculated by summing the coefficients of the Gaussian expansion, i.e., q A = 3 i=1 qi A .