  We present a surface stacking fault (SSF) energy approach to predicting
defect nucleation from the surfaces of surface-dominated nanostructure such as
FCC metal nanowires. The approach leads to a criteria that predicts the initial
yield mechanism via either slip or twinning depending on whether the unstable
twinning energy or unstable slip energy is smaller as determined from the
resulting SSF energy curve. The approach is validated through a comparison
between the SSF energy calculation and low-temperature classical molecular
dynamics simulations of copper nanowires with different axial and transverse
surface orientations, and cross sectional geometries. We focus on the effects
of the geometric cross section by studying the transition from slip to twinning
previously predicted in moving from a square to rectangular cross section for
$\ <100\ > /\{100\}$ nanowires, and also for moving from a rhombic to truncated
rhombic cross sectional geometry for $\ <110\ >$ nanowires. \hsp{We also
provide the important demonstration that the criteria is able to predict the
correct deformation mechanism when full dislocation slip is considered
concurrently with partial dislocation slip and twinning. This is done in the
context of rhombic aluminum nanowires which do not show a tensile reorientation
due to full dislocation slip.} We show that the SSF energy criteria
successfully predicts the initial mode of surface-nucleated plasticity at low
temperature, while also discussing the effects of strain and temperature on the
applicability of the criterion.
