The hard and soft acids and bases principle

Making use of the approximate expression for the total energy E of a system, in terms of the chemical potential mu and the hardness eta, E[rho] = N-e mu - 1/2N(e)(2) eta + E-core[rho], where N-e is an effective number of valence electrons and E-core[rho] represents the core contribution to the total energy, it is shown that the interaction between species whose softnesses are approximately equal is energetically favored, relative to the interaction between species whose softnesses are very different from each other.