  We use dispersion-corrected density-functional theory to determine the
relative energies of competing polytypes of bulk layered hexagonal
post-transition-metal chalcogenides, to search for the most stable structures
of these potentially technologically important semiconductors. We show that
there is some degree of consensus among dispersion-corrected
exchange-correlation functionals regarding the energetic orderings of
polytypes, but we find that for each material there are multiple stacking
orders with relative energies of less than 1 meV per monolayer unit cell,
implying that stacking faults are expected to be abundant in all
post-transition-metal chalcogenides. By fitting a simple model to all our
energy data, we predict that the most stable hexagonal structure has P$6_3$/mmc
space group in each case, but that the stacking order differs between GaS,
GaSe, GaTe, and InS on the one hand and InSe and InTe on the other. At zero
pressure, the relative energies obtained with different functionals disagree by
around 1-5 meV per monolayer unit cell, which is not sufficient to identify the
most stable structure unambiguously; however, multi-GPa pressures reduce the
number of competing phases significantly. At higher pressures, an AB$'$-stacked
structure of the most stable monolayer polytype is found to be the most stable
bulk structure; this structure has not been reported in experiments thus far.

Materials Science