  Unfolding the band structure of a supercell to a normal cell enables us to
investigate how symmetry breakers such as surfaces and impurities perturb the
band structure of the normal cell. We generalize the unfolding method,
originally developed based on Wannier functions, to the linear combination of
atomic orbitals (LCAO) method, and present a general formula to calculate the
unfolded spectral weight. The LCAO basis set is ideal for the unfolding method
because of the invariance that basis functions allocated to each atomic species
are invariant regardless of existence of surface and impurity. The unfolded
spectral weight is well defined by the property of the LCAO basis functions. In
exchange for the property, the non-orthogonality of the LCAO basis functions
has to be taken into account. We show how the non-orthogonality can be properly
incorporated in the general formula. As an illustration of the method, we
calculate the dispersive quantized spectral weight of ZrB2 slab and show strong
spectral broadening in the out-of-plane direction, demonstrating the usefulness
of the unfolding method.
