On the structure and physical origin of the interaction in H2...Cl- and H2...Br- van der Waals anion complexes.

The ab initio three-dimensional potential energy surface (PES) for the weak interaction of hydrogen molecule with bromine anion is presented. The surface was obtained by the supermolecular method at the coupled cluster with single and double excitations and noniterative correction to triple excitations (CCSD(T)) level of theory. Our calculations indicate the van der Waals (vdW) system for the linear orientation at R=3.37 Å with a well depth of D_e=660.1 cm^-1. The presented PES reveals also transition state for the perpendicular orientation at R=4.22 Å with a barrier of 607.1 cm-1. The physical origin of the stability of vdW H₂...Br^- structure with respect to the H₂...Cl^- one was analyzed by the symmetry adapted perturbation theory based on the single determinant Hartree–Fock (HF) wave function. The separation of the interaction energy shows that the dispersion forces play slightly more important role in the stabilization of the vdW system with Br^- than with Cl^-. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]