Ab initio calculations at the MP2/6-31G** level have been carried out on the non-Watson-Crick nucleic acids pairs G<FONT FACE=Symbol>&middot;</FONT>U and U<FONT FACE=Symbol>&middot;</FONT>U to obtain the interaction energies and to see whether the derived values are comparable or not with the canonical G-C, A-T and A-U pairs. Optimized geometries of the pairs show that the structural parameters of the isolated bases differ very little on pairing. The guanine -NH2 group does not participate in the hydrogen bonding formation and possesses a pyramidal structure; its intrinsic nonplanarity plays an important role in the out-of-plane intermolecular interactions. Thus, the G<FONT FACE=Symbol>&middot;</FONT>U pair projects three hydrogen bonding acceptor sites, namely, N7(G), O6(G) and O4(U) to the RNA major groove. The interaction energy (DHºint) calculated for the G<FONT FACE=Symbol>&middot;</FONT>U pair (-13.6 kcal/mol) is comparable to that determined for A-T (-13.0 kcal/mol), but considerable smaller than the experimental value reported for G-C (-21.0 kcal/mol). The U<FONT FACE=Symbol>&middot;</FONT>U pair follows the trend that pairing between pyrimidines bases should have lower interacting energies than purine-pyrimidine pairs