  We study the cross-section shape and size effects on the valley splitting in
PbSe nanowires within the framework of empirical $sp^3d^5s^*$ tight-binding
method.
  We consider ideallized prismatic nanowires, grown along [110], with the
cross-section shape varying from rectangular (terminated by $\{001\}$ and
$\{110\}$ facets) to rhombic (terminated mostly by $\{111\}$ facets).
  The valley splitting energies have the maximal value (up to hundreds meV) in
rectangular nanowires, while in rhombic ones they are almost absent.
  The shape dependence is shown to be similar for a wide range cross-section
sizes and different point symmetries of nanowires.
