An improved calculation of the transition state for the F + H₂ reaction

Using a large, balanced one-electron basis set, fully optimized reaction space (FORS) calculations to optimize the orbitals and to estimate the internal correlation energy, multi-reference configuration interaction calculations including all single and double excitations out of the FORS reference space to estimate a fraction of the external correlation energy, and the method of scaled external correlation (SEC), we calculate the interaction energy of F with H₂ in the vicinity of the saddle point for the reaction F + H₂ → HF + H. Our calculated barrier height, 1.6 kcal/mol, is considerably lower than values obtained in recent ab initio calculations, and the saddle point geometry is about 0.3 a₀ looser. This indicates that the part of the external correlation energy omitted from MR CISD calculations because of the incompleteness of the one-electron basis set and the truncation of the CI expansion, as estimated by the SEC method, has a significant effect on both the saddle point energy and its geometry.