Use of scaled external correlation, a double many-body expansion, and variational transition state theory to calibrate a potential energy surface for FH₂

A new potential energy surface is presented for the reaction F + H ₂ → HF + H. The regions of the surface corresponding to collinear and bent geometries in the F-H-H and H-F-H barrier regions are based on scaled external correlation (SEC) electronic structure calculations, and the F-H⋯H exit channel region is based on the previously developed surface No. 5. The functional form of the new surface includes dispersion forces by a double many-body expansion (DMBE), and the surface was adjusted so that the van der Waals well in the F⋯H-H region agrees with available experimental predictions. We have calculated stationary point properties for the new surface as well as product-valley barrier maxima of vibrationally adiabatic potential curves for F + H₂ → HF(υ′ = 3) + H,F + HD-HF(υ′ = 3) + D, and F + D₂ → DF(υ′ = 4) + D. The new surface should prove useful for studying the effect on dynamics of a low, early barrier with a wide, flat bend potential, as indicated by the best available electronic structure calculations.