Effect of spin contamination on the prediction of barrier heights by coupled-cluster theory:: F+H2→HF+H reaction

The role of spin contamination and quasidegeneracy in the energetics of a simple hydrogen abstraction reaction is studied by applying recently developed coupled-cluster (CC) methods to an exactly solvable model for the F + H-2 --> HF + H reaction [C. W. Bauschlicher and P. R. Taylor, J. Chem. Phys. 86, 858 (1987)]. To assess the role of spin contamination, the (spin)-unrestricted Hartree-Fock (UHF) CC with singles and doubles (CCSD) barrier height is compared with that obtained with a fully spin-adapted unitary group approach (UGA) based CCSD, while the quasidegeneracy effects are examined using the so-called reduced multireference (RMR) CCSD method. It is shown that the effect of spin contamination on the barrier height is more significant (0.7 kcal/mol) than that of the quasidegeneracy (0.2 kcal/mol), although the latter plays a more significant role for exothermicity. (C) 2000 John Wiley & Sons, Inc.