A COUPLED CLUSTER STUDY OF THE CLASSICAL BARRIER HEIGHT OF THE F+H2-]FH+H REACTION

Basis sets as large as F[7s7p5d4f2g]/H[6s5p4d2f] have been used in connection with the coupled cluster method including all single, double, and perturbative triple excitations [CCSD(T)] to predict the classical barrier height of the F + H-2 --> FH + H reaction. Employing transition state geometries optimized at the seven- and nine-electron CCSD (T) level with a [5s5p3d2f1g/4s3p2d1f] basis set, the calculated nine-electron barrier of 2.05 kcal/mol exhibits a 0.20 kcal/mol correlation contribution from the fluorine 2s electrons. Comparison with full configuration interaction (FCI) calculations in smaller basis sets for the electron affinity (EA) of fluorine, the energy of FH at stretched bond lengths, and the barrier height itself, demonstrate that the CCSD(T) method is capable of accurately reproducing the exact benchmark results. Employing large atomic natural orbital basis sets and correlating the F 2s electrons, the CCSD(T) predictions for the electron affinity (EA) of F (3.36 eV), the spectroscopic constants of FH (r(e) = 0.917 angstrom, omega-e, = 4146 cm-1, and D(e) = 140.9 kcal/mol), and the exothermicity of F + H-2 --> FH + H (31.6 kcal/mol), are in excellent agreement with the experimental values of 3.40 eV, 0.917 angstrom, 4138 cm-1, and 141.2 and 31.7 kcal/mol, respectively. The present CCSD(T) results are also in good agreement with previous seven-electron [5s5p3d2f1g/4s3p2d] predictions for the barrier height and exothermicity obtained at the multireference configuration interaction level of theory including the Davidson correction (MRCI + Q). They are also consistent with the seven- and nine-electron MRCI results, but do not support the + Q correction when the F 2s electrons are correlated. The largest CCSD (T) calculation in this work employs 209 basis functions in C2v symmetry and indirectly accounts for more than twenty-nine million connected triple excitations.