In our attempts to improve modeling of the χ potential, we used almost complete ribo- and deoxyribonucleoside models with the 5′-OH group replaced by a hydrogen (Figure 1; only the ribo compounds are shown). We omitted the 5′-OH group to avoid its contacts with the nucleobases (for instance, the contact of 5′-OH with H6 of pyrimidines), which would bias the parameters. Note that the value of the pseudorotation angle was fixed in all calculations (see below), and therefore, neglect of the anomeric effect of the missing 5′-OH group should not influence our results. We refer to the compounds in Figure 1 as ribo/deoxyribonucleosides or simply dN/rN hereafter to facilitate discussion, noting that in this work these terms always refer to the nucleosides with the 5′-OH replaced by a hydrogen. These molecules are probably the smallest models that could be reasonably used for our purpose as they include all the intramolecular contacts that occur upon rotation about the torsion angle. The intramolecular contacts are very important because they make major contributions to the torsion energy. For instance, the repulsive O4′···O2 and O4′···N3 contacts in purines and pyrimidines, respectively, correspond to the highest rotation barriers on the potential energy surface. Note also that increasing the complexity of the model beyond certain limits does not necessarily improve the quality of the results as some long-range interactions and contacts might introduce considerable additional problems.25 (link),52 As described below, to assess the influence of the sugar pucker, the calculations were performed for two sugar conformations in deoxyribonucleosides, C2′-endo and C3′-endo. For the ribonucleosides only the C3′-endo conformation was considered.