The W center in self-implanted silicon is the self-interstitial cluster   I_3
  We identify the W center in self-implanted crystalline Si with the three-membered self-interstitial cluster I_3 on the basis of first-principles density-functional-theory calculations matching all the known experimental signatures of the center (emission energy, extrinsic energy levels, activation energy and dissociation energy, local vibrational structure, and symmetry). 
G. M. Lopez, V. Fiorentini (Cagliari)