Polarizabilities and first hyperpolarizabilities of HF, Ne, and BH from full configuration interaction and coupled cluster calculations

Static and frequency-dependent polarizabilities and first hyperpolarizabili ties have been calculated for HF and Ne using full configuration interactio n (FCI) and a hierarchy of coupled cluster models: coupled cluster singles (CCS), an approximate coupled cluster singles and doubles model (CC2), coup led cluster singles and doubles (CCSD), an approximate coupled cluster sing les, doubles, and triples model (CC3), and coupled cluster singles, doubles , and triples (CCSDT). A previous study of BH concerning FCI benchmarking h as been extended to include CC3 and static CCSDT values. Systematic improve ments of the polarizabilities and the hyperpolarizabilities are found going from CCS to CCSD and from CCSD to CC3 or CCSDT. Little or no improvement o f the polarizabilities and no improvement of the hyperpolarizabilities are seen when going from CCS to CC2. The CCSD results represent a significant i mprovement over CCS and CC2 but are again surpassed by the CC3 results whic h agree very well with the FCI values. The relative error for the static po larizability at the CC3 level is 0.11% for Ne and, respectively, 0.16% and 0.20% for alpha(xx) and alpha(zz) of HF. For beta(zzz) and beta(zxx) the er rors are 0.50% and 1.7%, respectively. Only in the challenging case of BH d oes CCSDT improve the CC3 values. The dispersion for the polarizabilities a nd hyperpolarizabilities is predicted with increasing accuracy in the CCS-C C2-CCSD-CC3 sequence as expected from the increasing accuracy of the electr onic excitation energies. For all molecules the effect of orbital relaxatio n has been investigated for the static properties. The inclusion of orbital relaxation gives results that are somewhat different from the unrelaxed re sults but are in general no improvement. (C) 1999 American Institute of Phy sics. [S0021-9606(99)30228-2].