where {A, B} index over the molecules in the system whose centers of mass are located at xA , {j} indexes over each charged particle j in the molecule A, V AA coul is the intramolecular Coulomb potential in molecule A, and V AB coul is the intermolecular Coulomb potential between molecules A and B. Now to transform this into the dipole gauge, we use the PZW operator (Eq. 5), but now with Â(x A ) spatially varying inside the cavity (but still approximated as constant within a given molecule):