On the performance of CCSD(T) and CCSDT in the study of molecules with multiconfigurational character:: halogen oxides, HSO, BN and O3

Coupled cluster theory and Dunning's correlation consistent basis sets have been employed to determine the DeltaH(f.298)(0) of some halogen oxides XO, XOO and OXOX = F, Cl, Br. The estimated DeltaH(f.298)(0) of the XO radicals, OClO and OBrO are within +/-1 kcaVmol of the experimental results available. However those of ClOO and BrOO exhibit large errors when they are estimated with the atomization reaction. Our revisited DeltaH(f.298)(0),211 (FOO) including scalar relativistic effects and spin-orbit splitting is DeltaH(f.298)(0) (FOO) = 7.5 kcal/mol, 1-2 kcaVmol larger than the experimental results. For all the molecules considered CCSDT increased the binding energies, improving the agreement with the experimental estimations, except five molecules HSO, OClO, OBrO, BN (1)Sigma(+) and O-3. The correction of the CCSD(T) geometries for the missing triple excitations elongates the equilibrium bond lengths, improving the agreement with the experimental determinations for the XO radicals and both states of BN ((1)Sigma(+) and (3)Pi). The present results confirms that for some particular systems, such us the XO, XOO radicals, and hypervalent molecules like OFO, the CCSDT method is preferred over CCSD(T) to predict their thermochemistry and structural parameters. (C) 2004 Elsevier B.V. All rights reserved.