The history and the present state of the treatment of electron correlation is reviewed. For very small atoms or molecules calculations of higher than spectroscopic accuracy are possible. A detailed account for many-electron methods in terms of one-electron basis sets is given with particular attention to the scaling of computer requirements with the size of the molecule. The problems related to the correlation cusp, especially the slow convergence of a basis expansion, as well as their solutions are discussed. The unphysical scaling with the particle number may be overcome by localized-correlation methods. Finally density functional methods as an alternative to traditional ab-initio methods are reviewed. [Pg.185]

The multi-configmation linear response approach to electroweak quantum chemistry [106] therefore provides an efficient and systematic framework for an investigation of parity violating effects and their convergence with respect to basis set size and to electron correlation effects. Such a systematic procedure is almost invaluable for an estimate of the reliability of the theoretical predictions. [Pg.242]

The formulas derived in Cencek et al. [49] involve a simplifying approximation of only including one correlation term per basis function that is, only two electrons are correlated per function. In this work, all functions include correlation among all electrons. For two electrons there is no difference in the two forms of the basis, but for three or more electrons the formulas derived here should prove more efficient that is, they should converge faster and with a smaller basis set size. [Pg.463]

A number of fullerenes have been the subject of fully ab initio theoretical studies, and no attempt will be made here to review this work. However, for any but the smallest fullerenes these remain tremendously challenging computations due to the shear size of the molecules. Were it not for the extremely high icosahedral symmetry of buckminsterfullerene, most of the ab initio calculations which have been performed on it would still be impossibly time consuming even with modem computational resources. Even the largest of these, such as the TZP-MP2 (triple zeta plus polarization basis with electron correlation at the Moeller-Plesset 2nd order level) calculation on buckminsterfullerene of Haser, Almlof, and Scuseria [3], are still short of the basis set and correlation levels normally desired to be confident that the calculation is converged to chemical accuracy. As a result, semiempirical theoretical methods have played, and likely will continue to play, a major role in theoretical work on fullerenes. [Pg.535]

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