The optimized geometries of the H2O-H2O, H2O-HF, H2O-HCN, NH3-HF, HF-HF, NH3-HCN, H2O-NH3 and HCN-HCN hydrogen complexes were evaluated using B3LYP/6-31 1++G(d,p) calculations and parameters derived from Bader's atoms in molecules theory. Moreover, in accordance with the relation existent between the hydrogen bond energy and the increments in the bond length of the proton donor species, we present new proposals to estimate the hydrogen bond strength through the atoms in molecules calculations, as well as using traditional parameters such as atomic charge transfer and the infrared bathochromic effect of the acid molecules. (c) 2006 Elsevier B.V. All rights reserved.