  It can be argued that electron correlation, as a concept, deserves the same
prominence in general chemistry as molecular orbital theory. We show how it
acts as Nature's "chemical glue" at both the molecular and supramolecular
levels. Electron correlation can be presented in a general chemistry course in
an at least somewhat intuitive manner. We also propose a simple classification
of correlation effects based on their length scales and the size of the orbital
gap (relative to the two-electron integrals). In the discussion, we also show
how DFT can shed light on wavefunction theory, and conversely. We discuss two
types of "honorary valence orbitals", one related to small core-valence gaps,
the other to the ability of empty 3d orbitals in 2nd row elements to act as
backbonding acceptors. Finally, we show why the pursuit of absolute total
energies for their own sake becomes a sterile exercise, and why atomization
energies are a more realistic "fix point".
