ANALYTICAL REPRESENTATIONS OF ATOMIC PARTIAL CHARGES AND CHARGE FLUXES IN DISSOCIATING SYSTEMS

Atomic partial charges and their changes along dissociating coordinate s are investigated in the model systems HF, [HF...H]+, H2O, H3O+, H2O. ..HOH and [H2O...H...OH2]+. Ab initio calculations of dipole moment de rivatives as computational observables representing the partial charge s are presented. The results are fitted with an analytic distribution function of atomic partial charges that can capture the opposing limit ing behaviors of dissociating X-H bonds in both protonated and unproto nated cases. Specifically, it is found that a Fermi-Dirac type distrib ution function provides a reasonable economical representation for cha rge transfer across bonds. Atom types in dissociating systems are nece ssarily variables and an appropriate atom type function is introduced as a parameter of the analytic charge distribution function.