  With numerical simulations of the mW model of water, we investigate the
energetic stability of crystalline clusters for both Ice I (cubic and hexagonal
ice) and for the metastable Ice 0 phase as a function of the cluster size.
Under a large variety of forming conditions, we find that the most stable
cluster changes as a function of size: at small sizes the Ice 0 phase produces
the most stable clusters, while at large sizes there is a crossover to Ice I
clusters. We further investigate the growth of crystalline clusters with the
seeding technique and study the growth patterns of different crystalline
clusters. While energetically stable at small sizes, the growth of metastable
phases (cubic and Ice 0) is hindered by the formation of coherent grain
boundaries. A five-fold symmetric twin boundary for cubic ice, and a newly
discovered coherent grain boundary in Ice 0, that promotes cross nucleation of
cubic ice. Our work reveals that different local structures can compete with
the stable phase in mW water, and that the low energy cost of particular grain
boundaries might play an important role in polymorph selection.
