  We use first-principles density functional theory to study the generalized
stacking fault energy surfaces for pyramidal-I and pyramidal-II slip systems in
Mg. We demonstrate that the additional relaxation of atomic motions normal to
the slip direction allows for the appropriate local minimum in the generalized
stacking fault energy (GSFE) curve to be found. The fault energy calculations
suggest that formation of pyramidal-I dislocations would be slightly more
energetically favorable than that for pyramidal-II dislocations. The calculated
pyramidal-II GSFE curves also indicate that the full pyramidal II dislocations
would dissociate into the Stohr and Poirier (SP) configuration, consisting of
two $\frac{1}{2}\langle c+a \rangle$ partials, $\frac{1}{6}[11{\bar2}3] +
\frac{1}{6}[11{\bar2}3]$ , but the pyramidal-I GSFE curves, while also
possessing a local minimum, would not dissociate into the same SP
configuration. We report observation of these partials here emanating from a
$\{10{\bar1}2 \}$ twin boundary. Using MD simulations with MEAM potential for
Mg, we find that the full pyramidal-II $\langle c+a \rangle $ dislocation
splits into two equal value partials $\frac{1}{6}[11{\bar2}3] +
\frac{1}{6}[11{\bar2}3]$ separated by ~22.6 $\AA$. We reveal that the full
pyramidal-I $\langle c+a \rangle$ dislocation dissociates also into two equal
value partials but onto alternating $(30{\bar3}4)$ and $(30{\bar3}2)$ planes
with $\frac{1}{6} [20{\bar2}3]$ and $\frac{1}{6} [02{\bar2}3]$ Burgers vectors
separated by a 30.4 $\AA$ wide stacking fault. When a stress is applied, edge
and mixed dislocations of the extended pyramidal-II dislocation can move on
their glide plane; however, pyramidal-I dislocations of similar character
cannot.
