The CCSD(T) and CCSDT interaction energies were determined for model planar H-bonded complexes (formamide... form-formamide, formamidine... formamidine) and stacked complexes (ethylene... ethylene, formaldehyde... formaldehyde). Various basis sets from the 6-31G*(0.25) to aug-cc-pVDZ were used. Difference between CCSD(T) and CCSDT interaction energies were small and become negligible (bellow 0.1 kcal/mol) if the aug-cc-pVDZ (or aug-cc-pVDZ/cc-pVDZ) basis set was applied. This result strongly supports the use of the CCSD(T) method for determination of true stabilization energies of extended complexes. (C) 2004 Published by Elsevier B.V.