  We develop several configuration interaction approaches for characterizing
the electronic structure of an adsorbate on a metal surface (at least in model
form). When one can separate adsorbate from substrate, these methods can
achieve a reasonable description of adsorbate on-site electron-electron
correlation in the presence of a continuum of states. While the present paper
is restricted to the Anderson impurity model, there is hope that these methods
can be extended to ab initio Hamiltonians, and provide insight into the
structure and dynamics of molecule-metal surface interactions.
