The role of electron correlation in the five lowest energy isomers of C-36 fullerene is studied using the CASSCF method with single and multireference MP2 theories. It is shown that the distortion of the D-6h isomer at HF levels of theory is an artificial effect caused by the oversimplified form of the wavefunction. The singlet state of the D-6h isomer has diradical character, and it is the lowest energy state. The triplet states of the D-6h and D-2d isomers are 34 and 92 kJ/mol and the singlet state of the D-2d isomer is 148 kJ/mol higher in energy than the D-6h singlet state. (C) 2002 Elsevier Science B.V. All rights reserved.