Toward a chemisorption cluster model using the LCGTO‐X_α method: Application to Ni(100)/Na

A modified cluster approach for modeling local chemisorption phenomena is suggested on the basis of the linear combination of Gaussian‐type orbitals (LCGTO) X_α method. Contractions of the fitting bases are employed to take into account the reduced polarizability of a surface cluster and to access larger cluster sizes. Furthermore, embedding of a cluster in the surface is mimicked by Gaussian broadening of the one‐electron levels leading to fractional occupation numbers via a self‐consistently determined Fermi energy of the cluster. As a first application results are presented for the clusters Ni_nNa (n = 5, 9, 17) modeling the low coverage limit of the chemisorption system Ni(100)/Na. Calculated bond length, binding energy, and induced “surface” dipole moment show fair agreement with experimental values, indicating a substantial covalent character of alkali bonding on transition metal surfaces even in the zero coverage limit.