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 e2a0 2E h -1 and the anisotropy Δα≃ 11.58 e 2a0 2Eh -1 agree well with experimental measurements. A CCD value of γ̄ = 5.31 × 10 3 e4a0 4Eh -3 is obtained but a meaningful comparison with experiment is not possible. Further measurements of the second-dipole hyperpolarizability are definitely needed. © 1990 American Institute of Physics.