The Atomic Partial Charges Arboretum: Trying to See the Forest for the Trees

Abstract

Atomic partial charges are among the most commonly used interpretive tools in quantum chemistry. Dozens of different 'population analyses' are in use, which are best seen as proxies (indirect gauges) rather than measurements of a 'general ionicity'. For the GMTKN55 benchmark of nearly 2,500 maingroup molecules, which span a broad swathe of chemical space, some two dozen different charge distributions were evaluated at the PBE0 level near the 1-particle basis set limit. The correlation matrix between the different charge distributions exhibits a block structure; blocking is, broadly speaking, by charge distribution class. A principal component analysis on the entire dataset suggests that nearly all variation can be accounted for by just two 'principal components of ionicity': one has all the distributions going in sync, while the second corresponds mainly to Bader QTAIM vs. all others. A weaker third component corresponds to electrostatic charge models in opposition to the orbital-based ones. The single charge distributions that have the greatest statistical similarity to the first principal component are iterated Hirshfeld (Hirshfeld-I) and a minimal-basis projected modification of Bickelhaupt charges. If three individual variables, rather than three principal components, are to be identified that contain most of the information in the whole dataset, one representative for each of the three classes of Corminboeuf et al. is needed: one based on partitioning of the density (such as QTAIM), a second based on orbital partitioning (such as NPA), and a third based on the molecular electrostatic potential (such as HLY or CHELPG).

N‐Heterocyclic Silylenes as Ligands in Transition Metal Carbonyl Chemistry: Nature of Their Bonding and Supposed Innocence

“…The percentage AO contribution to MOs is based on gross Mulliken contributions . The Tolman electronic parameter (TEP) was calculated by simulating the IR‐spectra of [Ni(CO) 3 (NHC Me )] and [Ni(CO) 3 (NHSi Me )], respectively.…”

Pristine Graphene as a Racemization Catalyst for Axially Chiral BINOL

“…[37] We note that APT charges, which are based on dipole moment derivatives, have been shown to give a reliable picture of the molecular charge distribution. [38,39] Table S1 of the Supporting Information lists the APT charges on the hydroxyl oxygens and the carbon atoms connected to them in the reactant complex (RC) and catalyzed TS in the four solvents. Examination of these charges reveals that the partial positive charges on the carbon atoms and the partial negative charges on the oxygen atoms are reduced when moving from the RC to the TS.…”

DFT and Empirical Considerations on Electrocatalytic Water/Carbon Dioxide Reduction by CoTMPyP in Neutral Aqueous Solutions**

“…The use of small basis sets, which do not include dispersion components, allow an accurate calculation of partial charges using the Mulliken method and the results are similar to those obtained with the NBO (Natural Bonding Orbitals) method. [37] The basis set, def2-SVP, used in our calculations fits this definition and does not include dispersion components. Therefore, the partial charge values calculated by the Mulliken method are expected to be similar to those calculated by the NBO method.…”