The rhombohedral unit cells of alpha -boron crystals and of hypothetical al
pha -boron quasicrystals were investigated by applying ab initio quantum-ch
emical methods. The atomic decorations of such unit cells are generally bas
ed on a suitable arrangement of eight boron icosahedra, and thus the mechan
ism of their cohesion was simulated by relaxing a B-96 supercluster compose
d of icosahedral boron clusters located at the vertices of the rhombohedral
unit cells. Also, several alternative structures of B-96 clusters were pro
posed and compared to the rhombohedral phases. To this end, total energies
of those structures were computed on the basis of all-electron calculations
at the Hartree-Fock self-consistent field level of theory using the standa
rd STO3G basis set. Due to the large number of degrees of freedom, the geom
etries of all isomers were optimized under reasonable symmetry constraints,
but using analytical gradient methods. The resulting total energies show t
hat segments of nanotubes and quasiplanar sheets turn out to be more stable
than the unit cells of alpha -boron (quasi) crystals, indicating new kinds
of materials. (C) 2000 Academic Press.