It was shown, using ab initio total energy density functional theory, that so-called Wigner defects - an interstitial carbon atom right next to a vacancy - could exist in bundles of carbon nanotubes. Due to the geometrical structure of a nanotube, this defect had a formation energy that was lower than that of the vacancy itself; suggesting that it might be one of the most important defects that were created by electron or ion irradiation. They formed a strong link between nanotubes in bundles, and increased their shear modulus by a considerable amount; clearly indicating their importance to the mechanical properties of nanotube bundles.

Bundling Up Carbon Nanotubes through Wigner Defects. A.J.R.Da Silva, A.Fazzio, A.Antonelli: Nano Letters, 2005, 5[6], 1045-9