Exchange potential from the common energy denominator approximation for the Kohn-Sham Green's function: Application to (hyper)polarizabilities of molecular chains

An approximate Kohn-Sham (KS) exchange potential is developed based on the common energy denominator approximation (CEDA) for the static orbital Green's function. It preserves the essential orbital structure of the density response function. The response potential exhibites the characteristic step structure produced by the orbital densities. The CEDA density response function contains off diagonal steps produced by the orbital cross-products due to the complete exclusion of the occupied-occupied orbital mixing