Atomic point-charge potential

Tab. 1.3 Comparison between different sets of atomic point charges for a zwitterionic Gly-Ala dipeptide in aqueous solution. D-RESP electrostatic potential derived charges [12] fitted to all 36 configurations. Hirshfeld average value of the Hirshfeld charges [89c] along the full trajectory, Amber AMBER 1995 force field [86], Gromos GROMOS96 force field [85], The charges of equivalent atoms are imposed to be equal.

The potential energy of a core electron is not only affected by the charge of the atom of which it is a part, but is also affected by the charges of all the other atoms in the compound. That is, one must account for the work to remove the electron from the field of the surrounding charged atoms as well as from the atom which loses the electron. Both effects are accounted for by the so-called point-charge potential equation2 ... 

The theoretical potential function of a discrete charge distribution, i.e. of atomic point charges is given at point r as the following ... 

The first term always represents the exchange repulsion and the second term the London dispersion attraction. The Coulomb term is sometimes added in order to represent the electrostatic interactions among molecules, with fractional atomic point charges qx° and qxt> used. Alternatively, one has included these electrostatic contributions by adding the leading molecular multipole-multipole interaction term to an atom-atom potential of 12-6 or exp -6 type. 

Fig. 6. Electrostatic multipole expanded interaction energy between two ethene molecules for 8 different orientations at R = Ua (ab initio results from refs. - ). Fits and are atom-atom (point charge) model fits to the ab initio data is a fit with the charges shifted from the nuclei, has the charges fixed on the nuclei. Also indicated is an empirical atom-atom potential for details see ref. ...

Notwithstanding the last observation, the concept of atomic point charges is fundamental to chemical thinking and widely applied. In particular, the concept is central to molecular mechanics and dynamics simulations, where not only the electrostatic potential but also the electrostatic forces on nuclei must be satisfactorily reproduced. 

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