  For sparse materials like graphitic systems and carbon nanotubes the standard
density functional theory (DFT) faces significant problems because it cannot
accurately describe the van der Waals interactions that are essential to the
carbon-nanostructure materials behavior. While standard implementations of DFT
can describe the strong chemical binding within an isolated, single-walled
carbon nanotube, a new and extended DFT implementation is needed to describe
the binding between nanotubes. We here provide the first steps to such an
extension for parallel and concentric nanotubes through an electron-density
based description of the materials coupling to the electrodynamical field. We
thus find a consistent description of the (fully screened) van der Waals
interactions that bind the nanotubes across the low-electron-density voids
between the nanotubes, in bundles and as multiwalled tubes.
