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WIREs Comput Mol Sci
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Correct electrostatic treatment of noncovalent interactions: the importance of polarization

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The Hellmann–Feynman theorem assures us that the forces felt by the nuclei in a molecule or complex are purely classically electrostatic. Nevertheless, it is often claimed (incorrectly) that electrostatic considerations are not sufficient to explain noncovalent interactions. Such assertions arise largely from neglecting the polarization that is inherently part of the electrostatic interaction, and must be taken into account. Accordingly, we now outline the requirements for a correct electrostatic treatment and discuss the difference between physical observables and quantities that arise from mathematical models. Polarization and donor–acceptor charge transfer are shown to be equivalent for weak interactions. However, polarization is a physical observable while charge transfer, in this context, is mathematical modelling. We also discuss some popular schemes for analyzing noncovalent interactions. WIREs Comput Mol Sci 2015, 5:169–177. doi: 10.1002/wcms.1210 This article is categorized under: Electronic Structure Theory > Ab Initio Electronic Structure Methods Electronic Structure Theory > Density Functional Theory Molecular and Statistical Mechanics > Molecular Interactions
The MP2/aug‐cc‐pVDZ calculated MEP on the 0.001 a.u. isodensity surface of H3C―Cl polarized by a charge of −0.2692 at a distance of 3 Å from the chlorine atom along the extension of the C―Cl bond.
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Schematic diagram of the effect of polarization by a second water molecule on the dipole moment of water. The dipole moment is depicted by the red arrow with the negative pole at the arrowhead. (a) The situation as often discussed without considering polarisation. (b) A more complete model in which polarization of the donor water molecule is taken into account.
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Schematic depiction of the regions of a Fock‐matrix in a molecular‐orbital basis. The diagonal (black) elements contain the molecular orbital (MO)‐eigenvalues. Only mixing within the yellow occupied‐virtual block leads to a change in the electron density.
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Electronic density and density‐difference diagrammes: electronic density redistribution of H3C―Cl upon perturbation (a) by an array of point charges representing an H2O molecule (red indicates an increase in electron density, blue a decrease) and (b) in a fully quantum‐chemically described interaction with an H2O molecule [colour scheme as in (a)]. (c) The lowest unoccupied molecular orbital of H3C―Cl [the phases have been coloured to correspond to (a) and (b)]. The data for (a) and (b) are taken from Ref .
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A comparison of the MP2/aug‐cc‐pVDZ‐calculated MEP on the 0.001 a.u. isodensity surface of iodobenzene with that calculated from MEP‐fitted charges (i.e., the best possible atomic monopole fit to the MP2‐data). The colour scale only extends over the 0–10 kcal mol−1 region to emphasize the differences between the two plots.
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Electronic Structure Theory > Density Functional Theory
Electronic Structure Theory > Ab Initio Electronic Structure Methods
Molecular and Statistical Mechanics > Molecular Interactions

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