  We studied the electronic structure Bi(2)Se(3) employing density functional
theory. The calculations show that the Dirac states primarily consists of the
states at the interface of surface and sub-surface quintuple layers and the
emergence of the Dirac states depends on the surface terminations in sharp
contrast to their surface character expected for such systems. This manifests
the complexity of real materials and the realization of topological order in
real materials as an outstanding problem. We discover that the surface
character of the Dirac states can be achieved by adsorption of oxygen on
Bi-terminated surface due to the change in covalency by the relatively more
electronegative oxygens.
