 Although Coulson, Carroll, and Hoffmann defend the use of simple, idealized models to generate horizontal explanations, it is not clear that quantum calculations can never generate contrastive explanations. Although single vertical explanations are not contrastive, a theorist can conduct multiple calculations and in so doing, generate the information needed to make contrastive explanations. Many of the best examples of quantum chemistry have this character: a series of closely related calculations, attempting to get at chemically relevant trends.