  The structural and electronic properties of the wurtzite phase of the InAs
and GaAs compounds are, for the first time, studied within the framework of
Density Functional Theory (DFT). We used the full-potential linearized
augmented plane wave (LAPW) method and the local density approximation (LDA)
for exchange and correlation and compared the results to the corresponding
pseudopotential calculations.
  From the structural optimization of the wurtzite polymorph of InAs we found
that the c/a ratio is somewhat greater than the ideal one and that the internal
parameter u/c has a value slightly smaller than the ideal one. In the
all-electron approach the wurtzite polymorph has a smaller equilibrium volume
per InAs pair and a higher binding energy when compared to the zinc-blende
phase whereas the situation is reversed in the pseudo treatment. The energy
differences are, however, smaller than the accuracy of standard
density-functional codes (~30 meV) and a theoretical prediction of the relative
stability of the two phases cannot be made.
  In order to investigate the possibility of using an LDA calculation as a
starting point for many-body calculations of excitations properties, we here
also present the band-structures of these materials. The bands are calculated
with and without relativistic effects. In InAs we find that the energy gaps of
both polymorphs are positive when obtained from a non-relativistic calculation
and negative otherwise.
  For both semiconductors, we determine the spin-orbit splittings for the
zinc-blende and the wurtzite phases as well as the crystal-field splittings for
the new wurtzite polymorphs.
