UHF, spin-polarized DFT, and CASSCF calculations have been performed for a 2,5,8-tri-t-butylphenalenyl radical dimeric pair and its simplified model to elucidate the characteristics of chemical bonding and the origin of strong antiferromagnetic coupling. The results show that the SOMO-SOMO overlap interaction is responsible for the strong antiferromagnetic interaction and intermediate covalent-bonding between the phenalenyl radicals. The t-butyl groups in the three beta-positions hardly affect the magnetic coupling, simply playing steric hindrances in a crystalline state. Chemical indices can explain the bonding character and the efficiency of the electron correlation for the dimeric pair. (C) 2002 Published by Elsevier Science B.V.