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Molecular-modeling studies on the photochromism of 2-(2,4-dinitrobenzyl)pyridine (α-DNBP), a compound whose synthesis has been previously reported in this Journal, focus on the hydrogen atom migratory distances and the energies for the two very different tautomers arising from the photo-induced proton transfers. Students use these distances to determine the most likely sequence for tautomerization and to explain the relatively slow rate of reversion of the final photoproduct to the original form. They also measure the total energies of the original compound and the two tautomers and examine changes in the geometries that accompany the hydrogen atom migrations. The reason a non-photochromic form of α-DNBP can be found is explained by citing the specific geometric requirements for photochromism in the solid state of an optimal polymorph. The molecular modeling clearly demonstrates the importance of crystal geometric restraints in both the optimal polymorph and the two, diverse photoproducts arising from pho...