  Modeling deformation twin nucleation in magnesium has proven to be a
challenging task. In particular, the absence of a heterogeneous twin nucleation
model which provides accurate energetic descriptions for twin-related
structures belies a need to more deeply understand twin energetics. To address
this problem, molecular dynamics simulations are performed to follow the
energetic evolution of $\{10\overline{1}2\}$ tension twin embryos nucleating
from an asymmetrically-tilted grain boundary. The line, surface and volumetric
terms associated with twin nucleation are identified. A micromechanical model
is proposed where the stress field around the twin nucleus is estimated using
the Eshelby formalism, and the contributions of the various twin-related
structures to the total energy of the twin are evaluated. The reduction in the
grain boundary energy arising from the change in character of the prior grain
boundary is found to be able to offset the energy costs of the other
interfaces. The defect structures bounding the stacking faults that form inside
the twin are also found to possibly have significant energetic contributions.
These results suggest that both of these effects could be critical
considerations when predicting twin nucleation sites in magnesium.
