  A hybrid orbital model for the topological insulator Bi2Se3 is proposed and
compared to that of graphene. The existence of a pi-bond trimer on the Se1
layer at the van der Waals gap and on the surface is thought to be responsible
for the unusual surface conduction mechanism found in Bi2Se3 as a topological
insulator. The three pi-bonds are locked as permanent electric dipoles between
the Se1 layers in the van der Waals gap as an attractive force for the bulk as
a gapped semiconductor. In contrast, the pi-bond trimers on the surface are
unlocked and exhibit two degenerate quantum states, creating a local charge
transfer mechanism in the cross-bridge model that is responsible for the
surface conduction. The role of pi-bonds on the Bi2Se3 surface is compared with
that in graphene for a similar 2D band structure containing Dirac cones.
