Molecular modelling point charge mapping

The PCM method is one of the best known of such models. In essence, it involves the generation of a solvent cavity from spheres centered at each atom in the solute the polarization of the solvent is represented by means of virtual point charges mapped onto the cavity surface and proportional to the derivative of the solute electrostatic potential at each point, calculated from the molecular wavefunction. The point charges are then included into the one-electron Hamiltonian, and therefore they induce a polarization of the solute. An iterative procedure is performed until the wavefunction and the point charges are self-consistent. 

In this work the use of molecular electrostatic potential (MEP) maps for similarity studies is reviewed in light of the latest results. First, methods of obtaining MEP maps is overviewed. The methodology, reliability and the efficiency of calculations based on semi-empirical as well as ab initio methods are discussed in detail. Point-charge models and multipole expansion methods which provide MEP maps of satisfactory quality are evaluated critically. Later on, similarity indices of various kinds are analyzed, compared and examples of their use are shown. Finally, the last section lists and summarizes software packages capable of calculating MEP map based similarity indices. 

Figure 3-6. A diagram of mapping of the molecular system onto a grid in the PB model. The molecule represented with a fixed point charges is depicted in gray. The exterior is described by parameter k dependent on the ionic strength. The electrostatic potential is solved in the nodes of the 3D grid...

Such an expression has previously been used for comparative purposes, for the study of interaction between two molecular species, by computing the electrostatic potential of the first partner and by assuming some point charge model as representative of the charge distribution of the second partner. We also plan to extend this concept in a more subtle way by using an electron density contour map to describe the charge distribution of the second partner as a function of the space surrounding this second partner. 

Whereas Eq.(5.58) serves for the determination of local interactions between cluster models of a zeolite and interacting molecules, analytical expressions are needed for the interaction potential if one wishes to compute vibrational frequencies for purpose of comparison with experiment or if the potentials are to be used in Monte Carlo or molecular-dynamics simulation calculations. Sauer and co-workers developed such analytical potentials for the water-silica interaction system. The method makes use of the molecular electrostatic potential (MEP) maps and the functional form of EPEN/2 (Empirical Potential based on interactions of Electrons and Nuclei). EPEN/2 potential functions consist of a point-charge interaction term and... 

---