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

Christof Hättig and Poul Jørgensen
Department of Chemistry, Århus University, DK-8000 Århus C, Denmark

J. Chem. Phys. 109, 2762-2778 (1998).
(Received 18 February 1998; accepted 12 May 1998)

The recently developed coupled cluster cubic response approach is applied to the calculation of frequency-dependent second hyperpolarizabilities of Ne, Ar, N2 and CH4. A basis set investigation is performed for the static limits of the hyperpolarizabilities at the SCF and the coupled cluster singles doubles level (CCSD) using series of correlation consistent basis sets. For N2 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 Hatree-Fock, coupled cluster singles (CCS), CC2 and CCSD results for third harmonic generation. Dispersion curves are calculated at the CCSD level for the Kerr effect, degenerate four wave mixing, electric field induced second harmonic generation, and third harmonic generation. To get a realistic comparison with experiment we have taken into account zero point vibrational averaging, pure vibrational contributions and corrections for core correlation and connected triples. For all four examples we obtain much better agreement with experiment than in previous ab initio calculation.

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