Split charge equilibration method with correct dissociation limits

Analytic reactive potentials rely on electronegativity equalization to describe how the electron distribution is affected as chemical reactions occur. However, such models predict fractional charges for neutral species with different electronegativities. To overcome this well-known dissociation problem, an approach taking advantage of the concept of split charges [R. A. Nistor, J. G. Polihronov, M. H. Muser, and N. J. Mosey, J. Chem. Phys. 125, 094108 (2006)] is put forward. A first implementation is presented. Starting from a previous model [P. Bultinck, W. Langenaeker, P. Lahorte, F. D. Proft, P. Geerlings, M. Waroquier, and J. P. Tollenaere, J. Phys. Chem. A 106, 7887 (2002)], a new contribution to the total energy is introduced in order to make up for the lack of suitable constraints on the charge density. Its effect is to restrain charge transfer between remote atoms. As a consequence, systems in gas phase naturally decompose into neutral fragments. This result is achieved using two empirical parameters in addition to atomic electronegativities and hardnesses.