Population analysis is the study of charge distribution within molecules. The intention is to accurately model partial charge magnitude and location within a molecule. This can be thought of as a rigorous version of assigning partial charges on the atoms like chemists often do in Lewis dot structures. Partial atomic charges are not observable characteristics of molecules, and therefore the entire idea of modeling electron population is not unique. In order to assign charges to atoms, one must define the spatial region of those atoms, then add up all the charge in that region (integrate charge density over the volume). This may sound trivial, but where does one atom’s electron cloud end and the next begin? Which nucleus does a specific region of the electron cloud belong to? The process is arbitrary, but the results are useful.
