text: The calculated formation energies for the 3d transition met- 
  als are shown in Fig. 1. From the horizontal lines we see that 
  both divacancies are more stable than the monovacancy. This 
  may be attributed to the presence of a two-fold coordinated C 
  atom in the monovacancy, while all C atoms remain three-fold 
  coordinated in the divacancies. When a transition metal atom 
  occupies a vacancy, the strongest bonding to the C atoms is 
  through its d orbitals [26]. For this reason, Cu and Zn, which 
  both have ﬁlled d-bands, are rather unstable in the CNT. For 
  the remaining metals, adsorption in the monovacancies leads 
  to quite stable structures. This is because the three-fold coor- 
  dination of the C atoms and the CNT’s hexagonal structure are 
  recovered when the metal atom is inserted. On the other hand, 
  metal adsorption in divacancies is slightly less stable because 
  of the resulting pentagon defects, see upper panel in Fig. 1. A 
  similar behaviour has been reported by Krasheninnikov et al. 
  for transition metal atoms in graphene [21]. 