AN ABINITIO CONFIGURATION-INTERACTION STUDY OF THE REACTION BETWEEN SMALL LITHIUM CLUSTERS (LI4, LI-6) AND H-2 MOLECULE

A reaction path for the Li4 + H2 interaction has been determined allowing for the relaxation and the reconstruction of the system. For various steps along the reaction path characterized by the given values of one or two geometrical parameters the full geometry optimization of the remaining variables has been carried out in the framework of the Hartree-Fock (HF) procedure. The configuration interaction (CI) energies for the optimal HF geometries in each step have been calculated using multireference diexcited (MRD-CI) procedure. The calculated dissociation energy for the molecular hydrogen is reduced from 103.6 to 19.6 kcal/mol in the presence of Li tetrameter. During the first stage of the reaction path referred as "dissociative chemisorption" the geometry of the Li tetrameter does not change substantially. After dissociation of H2 the full reconstruction of the tetrameter takes place leading to a local minimum which is separated from the lowest minimum by a moderate barrier along the chosen reaction path. The stable compound has the form of deformed rhombus with two hydrogen atoms bridging two neighboring rhombus sides. The preliminary calculations using complete active space self-consistent field procedure followed by direct CI carried out for the Li6 + H2 interaction in the C2v symmetry yield fairly stable three dimensional LI6H2 compound, which can be formed after passing an energy barrier of comparable size at the present level of calculation as that found for the Li4 + H2 reaction.