Multiple hydrogen/proton transfer (MPT) reactions play a crucial role in a variety of important processes. The replication of DNA, one of the most fundamental biological processes, is prone to mutations, 1 which are created by complex steps involving MPTs. MPTs also have a prominent role in enzyme catalysis as well as in other non-biological systems such as ionic liquids and organic solids. Moreover, the transfer of protons can also be coupled to changes in the underlying electronic structure of the system as seen in proton-coupled electron transfer reactions (PCET). PCET reactions are ubiquitous and diverse. For example, excited state proton transfer, has potential applications for use as a fluorescent probe 14 in biological imaging and also for use as an organic opto-electronic material. Majority of the MPTs are found in organic molecules like DNA base pairs 17 and their analogues, 18-20 porphyrin, porphycene, phthalocyanines, and carboxylic acids where the transfer of two protons generate tautomers. This special category of proton transfer induced tautomerization is classified as double proton transfer (DPT). Depending on the manner in which the two hydrogen atoms transfer, the overall process can have two mechanism, namely, stepwise or concerted. Typically, a stepwise proton transfer involves H atoms/protons moving individually resulting in ionic intermediates. For example, Zewail and coworkers reported the hydrogen atoms of 7-azaindole dimers, 18 a prototypical system to the DNA base pair, transfer in a non-concerted manner generating ionic intermediate species. On the contrary, Kasha and coworkers interpret the transfer of protons to have a concerted mechanism where the protons are transferred simultaneously resulting in a single non-ionic transition state. The stepwise process involves charged intermediates and in most cases, have higher activation barriers whereas the concerted pathway suffers from the small kinetic pre-factor of two atoms transferring at the same time. The competition between the two processes determine the rate of tautomerization and the proportion of tautomers, which are essential in enzymatic reactions and drug delivery. The nuclei, due to their heavy mass, are typically treated as classical point like particles. However, recent studies have shown that the quantum fluctuations exhibited by them, either by the light ones like H or heavier ones like C and O, strongly influence the properties of many materials and different physical processes. Nuclear quantum effects (NQEs) in DPT are typically manifested through zero-point energy and tunnelling. While the zero-point energy fluctuations usually affect free energy barriers, tunnelling facilitates classically forbidden pathways at lower temperature. These manifestations in DPT are not only of fundamental interest in biological processes but are equally important in other chemical reactions. For example, Ivanov et al. and Wikfeldt et al. used path integral molecular dynamics (PIMD) simulations to capture NQEs in the dimer of formic acid and anti-ferroelectric squaric acid crystals respectively. They find the intermolecular DPT occurs through concerted proton transfer and is temperature dependent. Similarly, Cahlik et al. using a combination of low temperature STM, atomic force microscopy (AFM) and PIMD simulations, showed that the transferring hydrogen atoms in the one-dimensional quinonediimine molecular networks supported on Au(111) surface, are concerted. This promotes a delocalization of π-electrons along the molecular chain providing enhanced mechanical stability and giving rise to new states in the gap. In the study on intramolcular DPT in a single molecule of porphycene, Litman et al. using PIMD simulations report the hydrogen atoms tunnel concertedly at low temperatures (< 100 K) whereas a competition between the concerted and stepwise pathway is observed at high temperatures (100-300 K).