Electron-correlation effects on the static longitudinal polarizability of polymeric chains

Ab initio calculations of the static longitudinal polarizability of molecular hydrogen model chains have been performed at different levels of approximation to investigate the effects of including electron correlation. Unlike uncoupled and coupled Hartree-Fock calculations for which a split-valence atomic basis set already provides suitable longitudinal polarizability estimates, the techniques of the Mo/ller-Plesset partitioning leading to successive electron corrections, namely, MP2, MP3, and MP4, and the coupled-cluster ansatz including all double excitations, all single and double excitations, and all single and double excitations as well as a perturbational estimate of the connected triple excitations require at least additional polarization functions and a triple-ζ-type basis set in order to give suitable polarizability values. It has also been shown that including electron correlation decreases the longitudinal polarizability values and that the electron correlation effects are overemphasized when using a too small basis set. Within the Mo/ller-Plesset treatment of electron correlation, the relative importance of the different orders and the different classes of substitutions used in the intermediate states has been investigated. The double substitutions present the largest electron correlation correction to the coupled Hartree-Fock longitudinal polarizabilities per unit cell. If the atomic basis set is sufficiently extended, the third-order contribution is dominant.