Nuclear quantum tunnelling. Our simulations also allow for selectively understanding the role of nuclear tunnelling in DPT. The effect of nuclear tunnelling can be quantified by an increase in the radius of gyration of the ring polymer representing the quantum nucleus. The radius of gyration probes the quantum uncertainty in position of the nucleus and is proportional to the thermal de Broglie wavelength of that nucleus. Hence, it is expected that a proton transfer event due to tunnelling is to be accompanied with an enhancement of the radius of gyration of the proton. This enhancement in the delocalization of the hydrogen ring polymer during the course of tunnelling is known to occur in the low temperature region of paraelectric squaric acid and hexagonal ice. Therefore, to examine the extent of this effect in our study, we have computed the radius of gyration, r G =