  A long standing postulate in crystal plasticity of metals is that yielding
commences once the resolved shear stress on a slip plane reaches a critical
value. This assumption, known as Schmid law, implies that the onset of
plasticity is independent of the normal stress acting on the slip plane. We
examine the validity of this assumption in single crystal perfect lattices at
zero temperature by subjecting them to a wide range of combined normal and
shear stresses and identifying the onset of plasticity. We employ phonon
stability analysis on four distinct single crystal metals and identify the
onset of plasticity with the onset of an instability. Our results show
significant dependence of yielding on the normal stress, thereby illustrating
the necessity of considering non-Schmid effects in crystal plasticity. Finally,
contrary to the common assumption that instabilities in single crystals are of
long wavelength type, we show that short wavelength instabilities are abundant
in the nucleation of defects for a wide range of loading conditions.
