The convergence of some cluster models of hydrogen chemisorption

The ability of small substrate clusters to represent adequately the chemisorption of hydrogen by the semi-infinite substrate has been investigated for three models: (i) the Grimley-Pisani model (1974) of chemisorption by a tight-binding solid, (ii) the same model, but with self-consistency extended to the whole solid using Hubbard's Hamiltonian, and (iii) the CNDO model for chemisorption by Li metal. Fully converged results are known, by direct calculation for the semi-infinite substrate, only for model (i), where the present work shows in fact the slowest convergence rate with increasing cluster size. Extending the self-consistency as in (ii) and (iii) enhances the rate of convergence, but the CNDO model still needs at least eighteen Li atoms to present adequately the on-site chemisorption of hydrogen by (100)Li.