Pizzochero M Bonfanti M Martinazzo R 2019 Beyond the second row of the periodic table, the nature of the multiple bonds between the elements of the main groups remains yet elusive, and "non-classical" bonding schemes are often invoked for their description. Here, focusing on group 14, we have performed an accurate modeling of the Si-Si and C-C double bonds, including electron correlation effects. We have shown that Si[double bond, length as m-dash]Si bonds are "classical" and closely resemble C[double bond, length as m-dash]C ones, being similarly subjected to a sort of tug of war in which the σ bond favors distortion and the π bond opposes it. The essential difference between Si and C boils down to the sizes of their valence shells, which determine the π-bending stiffness. In carbon, such a stiffness is large because, upon bending, the atomic s orbitals interfere destructively with the p ones. In silicon, the s shell is smaller than the p one, the bending stiffness is reduced and the π bonds typically succumb, distort, and weaken. Electron correlation plays a major role in this context, since π bonds are far from their molecular orbital limit. Hence, we have further shown that upon weakening the effective repulsion between π electrons one may remove any structural instability, strengthen the π bonds and turn Si into a closer relative of C than it used to be. https://sonar.ch/global/documents/22469 eng info:eu-repo/semantics/altIdentifier/doi/10.1039/c9cp05192a info:eu-repo/semantics/altIdentifier/pmid/31782416 Physical chemistry chemical physics : PCCP. - 2019 Physical and Theoretical Chemistry General Physics and Astronomy To bend or not to bend, the dilemma of multiple bonds. http://purl.org/coar/resource_type/c_6501