A STUDY OF BASIS SET AND ELECTRON CORRELATION-EFFECTS IN THE ABINITIO CALCULATION OF THE ELECTRIC-DIPOLE HYPERPOLARIZABILITY OF ETHENE (H2C=CH2)

We report finite-field self-consistent field (SCF) and many-body perturbation theory calculations of the dipole polarizability and hyperpolarizability of ethene. Large Gaussian-type basis sets have been used to ensure the determination of reliable, near Hartree-Fock values for all the independent components of the dipole hyperpolarizability tensor gamma(alpha-beta-gamma-delta). The calculated SCF values of gamma display strong basis set dependence. Electron correlation changes drastically the SCF picture of gamma. The most important effect is the increase by nearly 70% of the component parallel to the double bond, gamma(ZZZZ). (molecular plane xz). It is found that the contribution from the fourth-order triple substitutions T4, dominates the fourth-order correction to the SCF value for almost all components of gamma. The best SCF values obtained in this work are (in atomic units), gamma(xxxx)=3466, gamma(yyyy)=11 080, gamma=3351, gamma(xxyy)=2390, gamma(yyzz)=2936, gamma(zzxx)=1660, and gammaBAR=6374e4a0(4)E(h)-3. The estimated electron correlation corrections (DELTA-corr) are 431 +/- 79, -870+/-421, 2318+/-87, -134+/-101, -67+/-133, 240+/-62, and 392+/-236e4a0(4)E(h)-3, respectively. Albeit electron correlation affects strongly the individual tensor components of gamma, the overall effect on the mean hyperpolarizability gammaBAR does not exceed a few percent. The final estimate of gammaBAR=(68 +/- 2) X 10(2) e4a0(4) E(h)-3 is in essential agreement with the experimental results of Ward and Elliott.