Affiliations: [a] Laboratoire de Chimie Théorique Appliquée, Facultés Universitaires Notre-Dame de la Paix, rue de Bruxelles, 61, B-5000 Namur, Belgium | [b] Laboratoire de Chimie Théorique et de Physico-Chimie Moléculaire, IPREM – ECP – UMR 5254, Université de Pau et des Pays de l'Adour, IFR, Rue Jules Ferry, BP27540 64075 PAU Cedex, France | [x] Department of Chemistry, University of Patras, Greece | [y] Department of Physics, University of Patras, Greece
Abstract: First and second hyperpolarizabilities of small silicon clusters have been calculated using conventional ab initio methods systematically increasing the amount of electron correlation. Besides Si5, upon successive addition of electron correlation in the MP2, MP3, MP4, CCSD, and CCSD(T) series, all clusters display the same behavior: i) the HF γ// values are the smallest, ii) the MP2 γ// values the largest, and iii) the latter values are good approximate to the reference CCSD(T) results because the overestimation is smaller than 10%. Contrary to the polarizability per Si atom, which decreases with the cluster size until reaching the bulk limit, the average second hyperpolarizability per Si atom presents a sawtooth behavior with maxima in γ// associated with even numbers of Si and minima with odd numbers of Si atoms.
