The Molecular Electrostatic Potential and Atomic Charges

The Moleariar Electrostatic PotentiaL The electric potential tf at a point P in space is defined as the reversible work per unit charge needed to move an infinitesimal test charge Q, from infinity to P, which we write as Woo- v/Qt- The SI unit of f is 

If our system consists of a single point charge located at (x, y, Za), then 4 i = QAl ohA) where r-iA is the distance between point A and point 1 with coordinates (xi, yi, zi). If our system consists of several point charges, then each contributes to f and 

To calculate x, y, z), one calculates an approximate electronic wave function for the equilibrium geometry (Section 15.11), uses (13.128) to calculate p x, y, z) from the approximate wave function, and uses (15.28) to calculate j). The MEP is found not to be strongly affected by the choice of basis set or by the inclusion of electron correlation. The MEP is often calculated at the HF/6-31G level of theory. 

When two molecules approach each other, the MEP of each plays a key role in their interaction. An electrophilic (electron-loving) species will preferentially attack a molecule at sites where the MEP is most negative. MEPs provide insight into molecular recognition processes such as enzyme-substrate and drug-receptor interactions. 

If we model each atom in a molecule as a sphere of radius equal to the van der Waals radius of the atom (for bonded atoms, these spheres overlap), the van der Waals surface of a molecule is defined by the outward-facing surfaces of these atomic spheres. In discussing intermolecular interactions, the MEP in the regions outside the van der Waals surface are most significant. 

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Section 15.8 The Molecular Electrostatic Potential and Atomic Charges 509... 

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