Density matrix treatment of localized electronic interactions in molecules and solids

A general method is presented for determining the electronic structure changes when an extended system is disturbed by a localized interaction. Such an interaction occurs, for example, when a small molecule is chemisorbed onto a metal surface. Our treatment is based on the density matrix formulation of the electronic structure problem and is applicable within the Hartree–Fock, Xα, or any other independent particle model. In essence, we obtain an effective self-consistent-field equation for the local interaction region. This equation includes charge transfer and all other delocalization effects due to the presence of the extended system. Illustrative calculations are carried out for an artificially modified beryllium atom and for chemisorption of atomic hydrogen onto a tungsten (100) surface. In the case of Be, the exact Hartree–Fock formulas are applied whereas, in the second illustration, we utilize the Anderson–Newns approximation.