  We report an ab-initio investigation of several possible Si and Ge pristine
nanowires with diameters between 0.5 and 1.2 nm. We considered nanowires based
on the diamond structure, high-density bulk structures, and fullerene-like
structures. We find that the diamond structure nanowires are unstable for
diameters smaller than 1 nm, and undergo considerable structural
transformations towards amorphous-like wires. Such instability is consistent
with a continuum model that predicts, for both Si and Ge, a stability crossover
between diamond and high-density-structure nanowires for diameters smaller than
1 nm. For diameters between 0.8 nm and 1 nm, filled-fullerene wires are the
most stable ones. For even smaller diameters (d~0.5 nm), we find that a simple
hexagonal structure is particularly stable for both Si and Ge.
