  This paper proposes a systematic generalized formulation for calculating both
atomic shuffling and shear candidates for a given compound twinning mode in
hexagonal closed-packed metals. Although shuffles play an important role in the
mobility of twinning dislocations in non-Bravais metallic lattices, their
analytical expressions have not been previously derived. The method is
illustrated for both flat planes and corrugated planes which are exemplified by
{11-22} and {10-12} twinning modes, respectively. The method distinguishes
between shuffle displacements and net shuffles. While shuffle displacements
correspond to movements between ideal atom positions in the parent and twin
lattices, net shuffles comprise contributions from shear on overlying planes
which can operate along opposite directions to those of shuffle displacements.
Thus, net shuffles in the twinning direction can vanish in a limiting case, as
is interestingly the case for those needed in the second plane by the b_4
dislocation candidate in {11-22} twinning. It is found that while shuffle
displacement vectors can be irrational when K_1 is corrugated, net shuffle
vectors are always rational.
