Ab initio calculations at the MP2 level have been carried out for two most stable neutral isomers of van der Waals dimer (CH3I)(2) and for their ionized forms. It was found within MP4(SDTQ)//MP2 approach that neutral head-to-tail HT isomer (dimerization energy -778 cm(-1)) is more stable than head-to-head HH form (dimerization energy -699 cm(-1)). Comparison of calculated ionization potentials and earlier experimentally observed appearance potential of dimer ion allowed us to conclude that ionization of the more stable neutral HT form is followed by conversion into the ionic HH+ form which is more stable than HT+ by 0.97 eV. (C) 2003 Elsevier B.V. All rights reserved.