  In the present work, we conduct large-scale orbital-free DFT calculations to
study the energetics of vacancy clustering in aluminum from electronic
structure calculations. The simulation domains considered in this study are as
large as those containing a million atoms to accurately account for both the
electronic structure and long-ranged elastic fields. Our results indicate that
vacancy clustering is an energetically favorable mechanisms with positive
binding energies for a range of vacancy clusters considered in the present
study. In particular, the $19$ vacancy hexagonal cluster lying in $\{111\}$
plane has a very large binding energy with the relaxed atomic structure
representative of a prismatic dislocation loop. This suggests that vacancy
prismatic loops as small as those formed from 19 vacancies are stable, thus
providing insights into the nucleation sizes of these defects in aluminum.
