Kohn-Sham studies of oxygen systems

The objective of this work was to use Kohn-Sham density functional methods to compute the enthalpies of formation of O-4, O-6, O-8, and O-12. To ascer tain the most effective functional combinations and basis sets for this pur pose, a variety of these were first tested by computing the energetics of a group of O-2 and O-3 reactions for which the relevant experimental data ar e available. On the basis of these results, the BLYP and BPW91 functionals were selected, in conjunction with the 6-31G* and 6-311G(2df) basis sets. T he calculated enthalpies of formation of O-4 varied between 92.6 and 97.2 k cal/mol, bracketing a reported CCSD(T) value of 95 kcal/mol. For O-6, O-8, and O-12, however, Kohn-Sham procedures were unable to consistently locate energy minima, even though Hartree-Fock and MP2 approaches do succeed in th is respect. It appears that any energy minima that may in reality exist for O-6, O-8, and O-12 are very shallow which is consistent with a conclusion reached earlier for O-4. (C) 2000 John Wiley & Sons, Inc.