First-principles examination of the twin boundary in hcp metals

We have performed total-energy electronic structure calculations to examine competing structures for the twin boundary, which can form under tensile stress along the c axis. The twin boundary structures are significantly different, most notably in their symmetry. These calculations show that, for all materials studied, the different structures have nearly the same energy, in agreement with calculations from empirical potentials. This result is surprising, given the difference in the structures, and the fact that previous first-principles calculations of the energies of compression twin boundaries have shown significant differences from empirical potentials.