  Atomistic simulations, based either on an empirical interatomic potential or
on ab initio calculations, are used to study the pyramidal glide of a 1/3
<1-210> screw dislocation in hexagonal close-packed zirconium. Generalized
stacking fault calculations reveal a metastable stacking fault in the first
order pyramidal {10-11} plane, which corresponds to an elementary pyramidal
twin. This fault is at the origin of a metastable configuration of the screw
dislocation in zirconium, which spontaneously appears when the dislocation
glides in the pyramidal plane.
