Coupled cluster calculations of the frequency-dependent second hyperpolarizabilities of Ne, Ar, N2, and CH4

The recently developed coupled cluster cubic response approach is applied to the calculation of frequency-dependent second hyperpolarizabilities for Ne, Ar, N-2, and CH4. A basis set investigation is performed for the static hyperpolarizabilities at the Hartree-Fock and the coupled cluster singles and doubles level (CCSD) using series of correlation consistent basis sets. For N-2 and CH4 we have employed basis set up to, respectively, 326 and 353 basis functions. The role of correlation effects on the dispersion of the hyperpolarizabilities is explored by a comparison of Hartree-Fock, coupled cluster singles (CCS), CC2 and CCSD results for the third harmonic generation. Dispersion curves are calculated at the CCSD level for the de Kerr effect, degenerate four wave mixing, electric field induced second harmonic generation, and third harmonic generation. For comparison with experiment we have considered zero point vibrational averaging, pure vibrational contributions and corrections for core correlation and connected triples. For all four systems we obtain much better agreement with experiment than in previous ab initio calculations. (C) 1998 American Institute of Physics.