Collective electronic oscillators for nonlinear optical response of conjugated molecules

The nonlinear optical polarizabilities of conjugated molecules are calculated and analyzed using a few collective electronic normal modes. A firm relationship between the optical response and ground state charge distributions and bonding network is established. The resulting physically intuitive picture relates the optical response directly to motions of charges in real space, identifies the origin of the scaling and saturation of optical nonlinearities with size, and has interference effects naturally built in. Drastic reduction in computational effort makes the present approach particularly attractive for computing and high order polarizabilities of large molecules.