(ii) Classical models need to be parametrized, in the simplest case (polarizable continuum model) with a macroscopic dielectric constant ϵ r or with various element or atom-specific quantities (Drude, Thole, charge equilibration). (iii) Most if not all classical models cannot be cast in an explicit form χ MM (r II , r ′ II ) that allows a straightforward evaluation of Eq. ( ).