Abstract: Ab initio calculations at the B3LYP/3-21G^**, HF/3-21G＞^** and ONIOM(HF/3-21G^** : AM1) levels of the theory in combination with counterpoise procedure for BSSE correction were performed on HMX dimers.There exist two O…H intermolecular contacts and the dispersion forces are dominant in the dimers.The corrected binding energies of the dimer are -15.10 and -17.81 kJ/mol at the HF/3-21G^** and ONIOM(HF/3-21G^**: AM1)levels, respectively.The calculation by the B3LYP method gives irrational corrected binding energies though it produces similar intermolecular distances as those produced by the HF or ONIOM method.The geometrical parameters, the contact distances and the binding energies demonstrated,for the first time, the validity of the ONIOM method applied in the calculation of the parameters of intermolecular interactions.

Key words: HMX dimer, Intermolecular interaction, Ab initio, ONIOM methodology