When dealing with only a single nucleobase, χ nad will be zero as there is no inter-fragment electron overlap. We can thus determine χ 0 by considering only a single nucleobase at it's equilibrium geometry with coordinates R 0 and imposing that the total density ρ(r) reduces to our reference density ρ 0 (r) so that ∆n = 0. From our minimum energy procedure, this gives us the following condition for χ 0 ,