McWeeny

McWeeny Atomic Partitioning

McWeeny Atomic Partitioning (often referred to simply as McWeeny partitioning) is a method used in quantum chemistry to partition the electron density of a molecule into atomic contributions. It aims to assign a portion of the total electron density to each atom within the molecule, allowing for the analysis of atomic charges and other properties.

The partitioning is typically based on a density matrix, which describes the probability of finding an electron at a particular point in space. The McWeeny procedure divides this density matrix into atomic contributions based on the basis functions centered on each atom. The method relies on integrals involving basis functions associated with different atoms.

While not unique or universally accepted, McWeeny partitioning offers a way to bridge the gap between the delocalized nature of electrons in molecules and the concept of localized atomic properties. The resulting atomic charges, often called McWeeny charges, can provide insights into the polarity of bonds and the distribution of electron density within the molecule. These charges are sensitive to the choice of basis set and the level of theory used in the underlying quantum chemical calculation.

Alternative methods for atomic partitioning exist, such as Mulliken population analysis, Löwdin population analysis, and Bader's Atoms in Molecules (AIM) theory, each with its own strengths and weaknesses. The choice of partitioning scheme can influence the interpretation of atomic properties and chemical bonding.