Electron correlations in molecules. III. Strength of electron correlations in localized and aromatic bonds of main-group atoms

Abstract

We investigate electron correlations within a minimal basis of various chemical bonds formed by second- and third-row atoms and hydrogen. The effects of correlations are characterized by two quantities: (i) intrabond correlation energy ϵ corr and (ii) the correlation strength parameter Σ. The latter describes the relative suppression of the charge fluctuations within a bond. The quantities ϵ corr and Σ can be associated with chemical bonds due to the local nature of the correlation phenomenon. It is found that one may distinguish between different classes of chemical bonds which are characterized by similar values of Σ parameter within each class. For instance, all considered bonds between hydrogen and main-group atoms fall into one class. The homopolar bonds formed by the atoms of the third row are characterized by a weaker σ but stronger π correlation strength than the respective bonds of the second row. The strongest correlations are found in homopolar single π bonds of third-row atoms. A particular detailed discussion is given of heteropolar bonds. The electron correlations of aromatic π electrons are found to be considerably weaker than those within the corresponding localized bonds. For the SCF input of the correlation calculation a semi-empirical method is used. The bond orbital approximation allows for a transparent interpretation of the computational results.