The quantum mechanical basis of conceptual chemistry

An experimentalist approaching theory for an understanding of conceptual chemistry that can be related to measurable properties, focuses on the electron density distribution. One finds in the topology of the electron density the definition of an atom, of the bonding between atoms, and of the boundary condition for the extension of quantum mechanics to an open system - to an atom in a molecule. This paper describes this approach, as it evolved from the failure of existing models to a study of molecular charge distributions and of how these studies resulted in the extension of quantum mechanics to an open system using the action principle.