Pterins are a family of heterocyclic compounds found responsible for many metabolic reactions and found as building blocks of important molecules such as flavins or folic acid. Theoretical simulations by Canuto and coworkers have shown that pterin can undergo a keto-enol equilibrium via a proton transfer in water to a nearby amine. Using MP2 combined with Monte-Carlo simulations of the solvent, they determined that the keto form is more stable than the enolic form (∆G=-11.21 kcal•mol -1 for the water assisted and ∆G=-16.36 kcal•mol -1 for the intramolecular). In addition, the water-assisted reaction had a lower activation free energy (∆G ‡ =6.67 kcal•mol -1 ) than the intramolecular proton transfer (∆G ‡ =32.55 kcal•mol -1 ). Due to the lack of experimental data, we assess the accuracy of the DFTB/MM-MD method to the MP2 results of Ref. 49.