ICS Award
Electron Correlation: Nature’s Strange and Wonderful Chemical Glue

Electron correlation corresponds to instantaneous evasive action between electrons. Its importance increases with the electron density, and hence as atoms are brought together in a molecule, or monomers into a dimer.

While it constitutes only a fraction of a percent of the total energy, it accounts for one-quarter to one-third of binding energies even in common organic molecules: in molecules with near-degeneracies, electron correlation may actually turn a metastable molecule into a stable one (e.g., in F₂, O₃, F₂O₂, ClF₅).

In our driving analogy, density functional theory (DFT) represents clear windows but a "near-sighted driver", perturbation theory a "far-sighted driver", and coupled cluster theory correct vision at all distances—successive levels of coupled cluster theory correspond to increasing numbers of simultaneously interacting cars. Unfortunately, it has a cost that rises steeply with the number of electrons: fortunately, this problem can be mitigated through layered basis set extrapolation and the introduction of explicit interelectronic distance terms (geminals).

At van der Waals distances between molecules, electron correlation manifests itself as dispersion, a major source of intermolecular attraction.