The barrier topography of the H+F-2 potential energy surface

The potential energy surfaces (PESs) for the electronic ground state and th e three lowest excited states of the H + F-2 reaction have been studied by ab initio multireference singles and doubles configuration interaction (MRC I-SD) calculations. Our main focus is laid on the description of the angula r dependence of the barrier region on the ground state surface. In general, except for the cases of increased symmetry, the barrier arises from avoide d-crossing interactions of the two lowest (2)A' states. In the collinear ca se it is the interaction between the two lowest Sigma states which is respo nsible for the barrier. Due to the weak interaction of these well-separated and not even neighboring states the barrier is relatively low. For broadsi de approach of H onto F-2 in C-2v symmetry the avoided crossing transforms into a symmetry induced conical intersection between the lowest (2)A(1) and B-2(1) states. With our largest calculation we obtain a height of 0.130 eV (0.078 eV including the Davidson correction) for the collinear barrier, an d of 1.12 eV (0.86 eV) for the broadside barrier at the conical intersectio n. We also consider semiempirical PESs frequently used for simulations of t he H + F-2 reaction. In comparison to our MRCI-SD results, a diatomics-in-m olecules PES shows a qualitatively different angular dependence of the barr ier in the broadside range. The implications of the barrier topography for the interpretation of experimental findings are discussed.