Paradigm pre-reactive van der Waals complexes:: X-HX and X-H2 (X = F, Cl, Br)

This review describes recent progress in ab initio calculations and modeling of weak pre-reactive van der Waals complexes that appear in the entrance channels of benchmark atom - diatom reactions. Examples from recent work are used to demonstrate how relevant potential energy surfaces are obtained and modeled from first principles. The paradigm complexes include the X(P-2) - HX and X(P-2) - H-2 (X = F, Cl, Br) systems, with O(P-3) - HCl included for comparison. In these complexes an interaction with either the HX or the H-2 molecule splits a degenerate P state of an open-shell atom into three potential energy surfaces, two of which are of the same symmetry. Application of state-of-the art highly correlated methods, CCSD(T) and MRCI, to the evaluation of adiabatic and diabatic states is discussed. Nonadiabatic coupling involving potential surfaces is compared for a number of complexes. Computational modeling of this term and its relationship to electrostatic interaction are also described. Spin - orbit coupling is shown to have dramatic effects on the structural and dynamic properties of these complexes.