Understanding and Interpreting Molecular Electron Density Distributions

This paper gives a simple pictorial introduction to the interpretation of electron densities to obtain information about bonding. The electron density of a molecule can be readily calculated using ab initio or density functional theory methods and it can also be obtained experimentally by X-ray crystallography. Unlike an orbital model of a molecule, the electron density is a physical observable. There are therefore advantages in interpreting the electron density to obtain information about bonding that are not as widely appreciated as they deserve to be. We give a simple introduction to the quantum theory of atoms in molecules (AIM) and its analysis of the electron density. We show how it provides a clear, rigorous, and unambiguous definition of an atom in a molecule that can be used as the basis for calculating the charge of the atom and indeed any of its other properties. We also show that familiar concepts such as ionic and covalent character cannot be rigorously defined or measured, but they can be re...