Localized-density-matrix method and nonlinear optical response

The linear scaling localized-density-matrix (LDM) method is generalized to calculate the nonlinear optical responses of large polyacetylene oligomers. The ground state reduced single-electron density matrix is initialized by the divide-and-conquer approach and is obtained subsequently using the LDM method. The third-harmonic generation spectra of large oligomers are calcul ated. The saturation of off-resonant second hyperpolarizability chi((3))(0) has been reinvestigated, and the value of chi((3))(0) is found to depend m ainly on the optical gap and the number of double bonds. (C) 2000 American Institute of Physics. [S0021-9606(00)30528-1].