 The purpose of many exercises in chemical modeling is to learn about real systems. In these cases, the model must bear certain relationships to real-world systems. But these relationships needn’t always be of extremely high fidelity. For example, Linus Pauling (1939) and early proponents of the simple valence bond model believed that this model captured the essential physical interactions that give rise to chemical bonding. This method is closely related to Lewis’ conception of bonding, treating molecules as composed of atomic cores (nuclei together with inner-shell electrons) and valence electrons which give rise to localized bonds. It stands in contrast to the molecular orbital method, which doesn’t localize the bonding electrons to any particular part of the molecule. Modern quantum chemists think of the valence bond model as a template for building models of greater complexity. Thus if a modern quantum chemist deploys the simple valence bond model to study a real molecule, she does so with a much lower standard of fidelity than Pauling would have. Her use of the model is only intended to give a first approximation to the most important features of the system.