Chemical potential equilibration models have proven to be a promising approach for describing charge transfer and polarization in the context of classical force fields. These have generally been justified in an ad hoc manner and are known to behave incorrectly in certain cases, presenting a stumbling block to widespread application. In this paper, we present a new wave function-based derivation of a CPE-like model, shedding some light on the nature of the approximations that are made. The concept of a pairwise hardness arises naturally from this derivation, leading us to suggest a model that employs a pairwise electronegativity. We show that this leads to a CPE-like model that dissociates correctly for a diatomic and furthermore predicts charges in agreement with ab initio methods for a simple diatomic.