Interaction energies of selected H-bonded nucleic acids base pairs, formamide dimer, and formamidine dimer have been evaluated using the second-order Moller-Plesset method (MP2) with extended basis sets of atomic orbitals. Basis sets with two sets of d-polarization functions on each non-hydrogen atom give the same interaction energies as medium-sized 6-31G** and 6-31G*(0.25) basis sets. This is due to a compensation of errors in the evaluation of the Hartree-Fock and correlation interaction energies. With a further increase of the size of the basis set, a steep improvement (increase in absolute value) of the interaction energy appears. It amounts to 0.6-1.1 kcal/mol with the cc-pVTZ basis set. The aug-cc-pVQZ. and cc-pV5Z, basis sets improve the interaction energy of model complexes by additional ca. 1.3 kcal/mol while these data appear to be very close to the basis set limit. In contrast to the MP2 method, Becke3LYP density functional theory method does not show any systematic improvement of the interaction energies with the increase of the basis set size. The previously published reference values for interaction energies of H-bonded base pairs obtained with medium-sized basis sets of atomic orbitals are likely to be underestimated tin absolute value) by 2.0-2.5 kcal/mol while relative stabilities of base pairs are correctly reproduced. Nevertheless, with the present computer facilities we cannot yet investigate in detail the influence of the quality of the optimized geometries of the dimers on their stabilization energies. It is expected, however, that this contribution is considerably smaller compared to the primary basis set enters evaluated in this study.