Anharmonicity and NLO responses: an exact diagonalization study

We present the exact numerical diagonalization of the Mulliken donor-accept or (DA) dimer with Holstein coupling. The resulting eigenstates are introdu ced in sum-over-states expressions of static optical susceptibilities. The careful partitioning of the sum, and the comparison with spectral propertie s give important clues on the role of electron-phonon (e-ph) coupling. Anha rmonicity does not appreciably affect vibrational spectra. nor linear elect ronic spectra, and is irrelevant for the static linear polarizability. By c ontrast, huge anharmonic contributions to hyperpolarizabilities are found: the harmonic approximation is unreliable for the calculation of non-linear responses, even for systems where it hardly affects linear optical spectra. (C) 2001 Elsevier Science B.V. All rights reserved.