  The electronic structure of four polytypes (beta, gamma, delta and epsilon)
of hexagonal GaSe and InSe is calculated from first principles, using the
WIEN2k and VASP codes and PBEsol prescription for the exchange-correlation
potential, aiming specifically at elucidating the crystallographic parameters
and comparing the energy placement of corresponding competing structures.
Further on, the compounds with different composition of the same constituents,
namely the ordered-vacancies systems Ga2Se3 and In2Se3, were subject to a
similar study, of which the relaxed crystal structure data for three different
phases is reported. Comparison is done with the nomi- nal wurtzite structure
over which the cation vacancies are introduced, and the relaxation pattern
discussed.
