Note that if the system is composed of more than two subsystems, higher order terms (as many as the number of subsystems) need to be taken into account, and Eq. ( ) is then a one-body screening correction approximation. It includes a modified Coulomb interaction ṽ I-I C , in which the term v I-I reac = v I-II C χ II v II-I C can be interpreted as a reaction field, describing the fluctuation of a charge at a point r due to a charge fluctuation at r ′ both in subsystem I which is mediated by the polarization of subsystem II. With the help of the frequency-dependent dielectric function of subsystem I as