  We compute atomistically the heat conductance for ultra-thin pristine silicon
nanowires (SiNWs) with diameters ranging from 1 to 5 nm. The room temperature
thermal conductance is found to be highly anisotropic: wires oriented along the
<110> direction have 50-75% larger conductance than wires oriented along the
<100> and <111> directions. We show that the anisotropies can be qualitatively
understood and reproduced from the bulk phonon band structure. Ab initio
density functional theory (DFT) is used to study the thinnest wires, but
becomes computationally prohibitive for larger diameters, where we instead use
the Tersoff empirical potential model (TEP). For the smallest wires, the
thermal conductances obtained from DFT- and TEP calculations agree within 10%.
The presented results could be relevant for future phonon-engineering of
nanowire devices.
