Commensurability effect on the electronic structure of carbon nanostructures: Impact on supercell calculations in nanotubes

Friedel oscillations are ubiquitous features seen in all impurity-doped metallic structures but in the case of graphene-like materials they are not so evident because the relevant wavelengths are perfectly commensurate with the lattice parameter. Here we demonstrate that this commensurability effect leads to a slow convergence of supercell-based total energy calculations in impurity-doped carbon nanotubes. We derive a mathematically transparent expression for the formation energy and identify a very distinctive dependence on the size of the supercell unit. We make use of this dependence through a simple extrapolation scheme to obtain density functional theory results with accuracy levels that would otherwise require enormously large unit cells.