Interactions in Open-Shell Clusters:  Ab Initio Study of Pre-reactive Complex O(3P) + HCl

Van der Waals interactions between the ground-state triplet O(3P) atom and the closed-shell HCl molecule are investigated in the pre-reactive region. Three adiabatic (two of A‘ ‘ symmetry and one of A‘ symmetry) and four non-relativistic diabatic potential energy surfaces are obtained by combining a restricted open-shell coupled cluster approach with the multireference configuration interaction method. The lower A‘ ‘ adiabatic potential surface has a single minimum (D e = 589 cm-1) for a linear O···HCl configuration. The upper A‘ ‘ potential has a weak (D e = 65 cm-1) minimum for a linear HCl···O configuration. The A‘ adiabatic potential has a weak (124 cm-1) T-shaped minimum. Adiabatic potentials intersect once in the O···HCl linear configuration and twice in the linear HCl···O geometry. The role of electrostatic interactions in shaping these potentials is discussed. The effects of spin−orbit coupling on this interaction are also investigated assuming a constant value of the SO parameter.