  The recent discovery of pure boron nanotubes raises questions about their
detailed atomic structure. Previous simulations predicted tubular structures
with smooth or puckered surfaces. Here, we present some novel results based on
ab initio simulations of bundled single-wall zigzag boron nanotubes (ropes).
Besides the known smooth and puckered modifications, we found new forms that
are radially constricted, and which seem to be energetically superior to the
known isomers. Furthermore, those structures might be interpreted as
intermediate states between ideal tubular phases and the known bulk phases
based on boron icosahedra.
