I. INTRODUCTION Within the framework of classical force fields, the number of computer experiments on sac- charides has grown considerably in recent years, and various systems have been addressed $^{1–27}$. Devising a realistic model of monosaccharides is obviously a decisive step in order for carbo- hydrates simulations to have enough predictive power. The accurate description of monosac- charides with classical force fields is not an easy task, because of the delicate interplay of different factors such as the presence of a high number of intramolecular hydrogen bonds, the competition of these hydrogen bonds with water-sugar ones and important steric and electrostatic effects between ring substituents in spatial proximity (see for example Ref. 28 and references within). The problem of reproducing some carbohydrates peculiarities, such as the rotameric distribution of the hydroxymethyl group or the anomeric and exo-anomeric effects, have been addressed in various force fields, and the reader can find some comparative analyses in Refs. 29–31. However, considerably less attention has been devoted so far to the correct reproduction of the ring conformational properties. Cyclic monosaccharides can appear indeed as puckered rings, and their conformational transitions dramatically alter the equilibrium properties of both single sugar rings as well as those in oligo and polysaccharides $^{32}$. Despite the large number of possible puckered conform- ers, many biologically relevant monosaccharides in the pyranoid form appear almost always in one stable puckered conformation, the second most populated state being so unlikely not to be detectable by actual experimental techniques. In spite of that, several authors reported an inappropriate percentage of secondary puckered conformations 32–37 when modeling car- bohydrates using classical force fields such as $_{gromos}$34,35,38–41 or $_{opls-aa}$42 for simulations in solution. Regarding the latest gromos parameter set for carbohydrates (45a4) 35 (which incidentally was also tuned to improve the description of puckered conformers with respect to older versions), non-chair conformers have been shown to be accessible during equilibrium simulation runs of β -d- glucose, with a statistical frequency of 0.7% $^{28}$. A more quantitative analysis has been performed recently, by computing the free energy difference between ring conformers of β -d- glucose $^{43}$. The prediction of the inverted chair free energy, ≃ 16 . 5 kJ/mol, is almost 10 kJ/mol lower than most theoretical and ab-initio simulation results. A reparametrization of those force fields which present unphysical ring conformers at equilibrium would be therefore highly desirable. A quantitatively accurate description of 2