Anharmonicity and NLO responses: an exact diagonalization study

We present the exact numerical diagonalization of the Mulliken donor–acceptor (DA) dimer with Holstein coupling. The resulting eigenstates are introduced in sum-over-states expressions of static optical susceptibilities. The careful partitioning of the sum, and the comparison with spectral properties give important clues on the role of electron–phonon (e–ph) coupling. Anharmonicity does not appreciably affect vibrational spectra, nor linear electronic spectra, and is irrelevant for the static linear polarizability. By contrast, 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.