25 × 10 3 water molecules in a simulation box with an edge of approximately 9.6 nm, using a cut-off of 1.3 nm for every nonbonded interaction, and an integration timestep of 2 fs. Every other parameter and algorithm employed has been the same as in the metadynamics run discussed so far. Indeed, we observed the appearance of both boats and inverted chairs conformers, with a statistical frequency of 0.06% and 1.1%, respectively (see the upper panel of Fig. 6. These values are close to the one expected from the free energy calculations (see Tab. III). However, a look at the time evolution of the number of inverted chain in the simulation box (lower panel of Fig. 6) tells us that equilibrium has not been reached yet. Still, this result is to our opinion quite valuable, because it gives some information about the kinetics of ring conformational transitions in β -d- Glc, showing that it is not unlikely to observe inverted chair conformers in equilibrium simulations of conventional size, using the 45a4 parameter set. We performed similar simulations of other sugars, namely, β -d- Gal and α -d- Glc, and in both cases we observed the appearance of inverted chairs, although to a much smaller extent, showing that the kinetics of the chair to inverted chair transition is much slower in these cases (consistently with the results of Ref. 28). This fact is obviously related to the height of the barrier that separates $^{4}$C$_{1}$ conformers from other ones (see Tab. III), which appears to be lower in β -d- Glc than in all other cases. After realizing the difficulties of the 45a4 parameter set in reproducing puckering prop- erties of pyranoses, we planned to re-parametrize the force field, by finding a minimal set of changes that could fix at least the qualitative aspects discussed so far. At a first glance, this task could seem a bit intimidating, because puckering variables (and therefore the relative free energy landscapes) depend directly on all six ring atoms and also — indirectly, but possibly to a considerable extent — on the ring substituents. The number of parameters on which puckering free energy depends is, therefore, quite high. A completely automated procedure is out of question, and one needs to adopt an heuristic approach. To keep the re-parametrization as general as possible, and the number of parameters to be tuned low, we decided that our approach should adhere to the following criteria: (a) only parameters not directly involved in inter-molecular interactions should be tuned; (b) the B. Force Field Re-parametrization 18