Combining B3LY-P/6-311++G(d,p) calculations and topological parameters derived from atoms in molecules theory, such as electronic density and Laplacian field, the strengths of the inter-intramolecular hydrogen bonds in the C2H2-2HF and C2H4O-2HF trimolecular complexes were measured. This new way to quantify energetically these interactions is due to limitation of traditional concepts based on simple difference between the energy of the complex and that of its constituent fragments. (c) 2006 Elsevier B.V. All rights reserved.