Electronic kinetic energy decrease as two metallic parallel C nanotubes are brought together from infinity

By utilising the virial theorem, an expression is derived from the recent work of Dobson et al. for the initial decrease of electronic kinetic energy as two metallic parallel C nanotubes of equal diameter are brought together from infinity to a (still large) separation. To yield the dispersion energy proposed by Dobson et al. [J.F. Dobson, A. White, A. Rubio, Phys. Rev. Lett. 96 (2006) 073201], it is shown that it is the initial drop in the kinetic energy that is responsible for the long-range attractive interaction