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+H2→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(v′=3)+H,F+HD→HF(v′=3)+D, and F+D2→DF(v′=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.