text: is the total energy of the pristine nanotube with a physisorbed 
  transition metal atom. We have considered the monovacancy 
  and two divacancies shown in Fig. 1. The energy required to 
  form an empty vacancy is obtained from  where E[X@M@VC] is the total energy of molecule X on 
  a transition metal atom occupying a vacancy, and E[X] is the 
  gas phase energy of the molecule. 
  
  From the adsorption energies plotted in Fig. 2(a), we see 
  that the earlier transition metals tend to bind the adsorbates 
  stronger than the late transition metals. The latest metals in 
  the series (Cu and Zn) bind adsorbates rather weakly in the 
  divacancy structures. We also note that O2 binds signiﬁcantly 
  stronger than any of the three target molecules on Ti, V, Cr, 
  and Mn (except for Cr in divacancy I where H2S is found to 
  dissociate). Active sites containing these metals are therefore 
  expected to be completely passivated if oxygen is present in 
  the background. Further, we ﬁnd H2O is rather weakly bound 
  to most of the active sites. This ensures that these types of 
  sensors are robust against changes in humidity. 