A quantum mechanical-Poisson-Boltzmann equation approach for studying charge flow between ions and a dielectric continuum

This paper presents a theoretical model for the investigation of charge tra nsfer between ions and a solvent treated as a dielectric continuum media. T he method is a combination of a semiempirical effective Hamiltonian with a modified Poisson-Boltzmann equation which includes charge transfer in the f orm of a surface charge density positioned at the dielectric interface. The new Poisson-Boltzmann equation together with new boundary conditions resul ts in a new set of equations for the electrostatic potential (or polarizati on charge densities). Charge transfer adds a new free energy component to t he solvation free energy term, which accounts for all interactions between the transferred charge at the dielectric interface, the solute wave functio n and the solvent polarization charges. Practical calculations on a set of 19 anions and 17 cations demonstrate that charge exchange with a dielectric is present and it is in the range of 0.06-0.4 eu. Furthermore, the pattern of the magnitudes of charge transfer can be related to the acid-base prope rties of the ions in many cases, but exceptions are also found. Finally, we show that the method leads to an energy decomposition scheme of the total electrostatic energy, which can be used in mechanistic studies on protein a nd DNA interaction with water. (C) 2000 American Institute of Physics. [S00 21-9606(00)50607-2].