 There is a second reason for employing simple, more highly idealized models in chemistry, which stems from the explanatory traditions of chemistry. In developing this point, Hoffmann argues that there are two modes of explanation that can be directed at chemical systems: horizontal and vertical (Hoffmann 1997). Vertical explanations are what philosophers of science call deductive nomological explanations. These explain a chemical phenomenon by deriving its occurrence from quantum mechanics. Calculations in quantum chemistry are often used to make predictions, but insofar as they are taken to explain chemical phenomena, they follow this pattern. By showing that a molecular structure is stable, the quantum chemist is reasoning that this structure was to be expected given the underlying physics.