  We investigate the piezoresistive effect of carbon nanotubes (CNTs) within
density functional theory (DFT) aiming at application-relevant CNTs. CNTs are
excellent candidates for the usage in nano-electromechanical sensors (NEMS) due
to their small band gap at zero strain leading to a finite resistivity at room
temperature. The application of strain induces a band gap-opening leading to a
tremendous change in the resistivity. DFT with the LDA approximation yields
reasonable results for pure carbon systems like CNTs and is applied to
calculate the electronic structure of experimentally relevant CNTs. For the
transport part, a simple ballistic transport model based on the band gap is
used. We compare our DFT results for the band gaps of strained CNTs to results
of tight binding (TB) models. By introducing a scaling factor of $\sqrt{2}$, an
excellent agreement of the the DFT data with TB model published by Yang et al.
[1] is obtained.
