  Carbon nanotubes have properties depending on the arrangement of carbon atoms
on the tube walls, called chirality. Also it has been tried to grow nanotubes
of only one chirality for more than a decade it is still not possible today. A
narrowing of the distribution of chiralities, however, which is a first step
towards chirality control, has been observed for the growth of nanotubes on
catalysts composed of nickel and iron. In this paper, we have calculated
carbon-metal bond energies, adhesion energies and charge distributions of
carbon nanotube caps on Ni, Fe and NiFe alloy clusters using density functional
theory. A growth model using the calculated energies was able to reproduce the
experimental data of the nanotube growth on the alloy catalysts. The electronic
charge was found to be redistributed from the catalyst particles to the edges
of the nanotube caps in dependence of the chiral angles of the caps increasing
the reactivity of the edge atoms. Our study develops an explanation for the
chirality enrichment in the carbon nanotube growth on alloy catalyst particles.
