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(2P)–HX and X(2P)–H2 (X = F, Cl, Br) systems, with O(3P)–HCl included for comparison. In these complexes an interaction with either the HX or the H2 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.