How important is electron correlation for the hyperpolarizability of ethyne?

The dipole polarizability(α) and second dipole hyperpolarizability(γ) tensors are calculated for the ground state of ethyne (HCCH). An extensive basis set study is carried out at the finite-field self-consistent-field level. Accurate results based on the finite-field coupled-cluster double excitation model (commonly known as CCD) are also calculated. Electron correlation lowers the isotropic averages ᾱ and γ̄ by only 3.4% and 1.8%, respectively, although the components of the γ tensor are affected by amounts up to 13%. The CCD values of ᾱ≂ 22.52 e2a20E−1h and the anisotropy Δα≂11.58 e2a20 E−1h agree well with experimental measurements. A CCD value of γ̄ ≂ 5.31×103 e4a40E−3h is obtained but a meaningful comparison with experiment is not possible. Further measurements of the second-dipole hyperpolarizability are definitely needed.