Study of collective effects in the stability and chemisorption properties of finite lithium clusters

Abstract A multibody analysis of the stabilization energies and chemisorption capacities of clusters of lithium atoms that represent models of the (100) and (110) surfaces of the bcc crystals of metallic lithium is presented. All calculations of the interaction energies reported here are at the CI level (i.e. they include extensive configuration interaction). The stability of small Li n clusters ( n ⩽ 6) is shown to have important nonadditive contributions and the relative values of all of the few-body contributions (three-, four-, five- and six-body) are quite large for clusters up to six Li atoms. A similar situation is found for the multibody expansion of the Li n -H interaction energy, which is supposed to give a model of H atom adsorption in the Li (100) and (110) surfaces. We interpret these results as evidence for strong collective effects for both the stabilization of Li clusters (and solid Li) and for the chemisorption capacity of H atoms by a Li surface. Therefore it is evidently necessary to study nonadditive effects whenever trying to predict optimal or representative geometrical structures for Li metal particles. An even more decisive role is played by the multibody contributions to the Li n -H adsorption energies. Firstly the multibody expansion of these energies does not show any traces of convergency. Furthermore we show that those systems for which the nonadditive effects are relatively less important (i.e. which are nonadditively unsaturated ) systematically have larger chemisorption energies. Using this rule we are able to explain how and why the different geometrical and structural factors of each Li cluster determine the changes in their chemisorption capacities.