The geometries and the relative energies of three low-lying (ClO)(2) isomers in the ground state have been calculated using density functional theory (DFT) methods. The DFT relative energies using extended basis sets vary by as much as 4.5 cal/mol, depending upon the functionals employed, but differ by less than 2.5 cal/mol from the available CCSD(T) results with large atomic natural orbital basis sets. DFT calculated geometries of the isomers are also in reasonable agreement with experiment. The present study shows that the DFT methods could be used to investigate relative energetics of (ClO)(2) isomers, for which large basis sets are essential even for modest accuracy. (C) 1998 Elsevier Science B.V.