We present first-principles calculations for the thermal reaction in the photochromic molecular crystals of 2-(2',4'-dinitrobenzyl)-pyridine (DNBP). Under irradiation this dye molecule undergoes a characteristic change of color as a result of an intramolecular proton-transfer reaction. By simulating the thermal reaction with first-principles molecular dynamics, we show that during the thermal proton transfer a hydrogen bond from the o-nitro group to the migrating proton is formed intermediately, which serves as a "bridge" between the educt and product forms. Furthermore, we present the experimentally not accessible structures of the crystalline photoisomers. On the basis of our results we conclude that photoisomerization is possible in the crystalline phase. This property is of elementary interest for technical applications of this system.