  We have developed a pair-potential approach for the evaluation of van der
Waals interaction between carbon nanotubes in bundles.
  Starting from a continuum model, we show that the intertube modes range from
$5 cm^{-1}$ to $60 cm^{-1}$. Using a non-orthogonal tight-binding approximation
for describing the covalent intra-tube bonding in addition, we confirme a
slight chiral dependance of the breathing mode frequency and we found that this
breathing mode frequency increase by $\sim$ 10 % if the nanotube lie inside a
bundle as compared to the isolated tube.
