  Ab initio calculations were applied to large size slabs simulations for
determination of the properties of the principal structures of the
stoichiometric GaN(0001) surface. The results are different from published
previously: stoichiometric GaN(0001) surface structure is characterized by
mixed structure in which 3/8 top layer Ga atoms remain in standard position
with $sp^3$ hybridized bonding while the remaining 5/8 top layer Ga atoms is
located in plane of N atoms with $sp^2$ hybridized bonding. This involves
charge transfer to the previous one, entailing energy optimization, therefore
Ga-s and Ga-p energy difference is driving force for the transition. In large
size surfaces Ga atoms create (4 x 4) reconstruction which is additionally
stabilized by strain optimization. Heavy doping in the bulk changes $sp^3$ to
$sp^2$ ratio thus confirming the charge transfer mechanism of the
reconstruction. The charge transfer energy optimization is universal for all
surfaces terminated by $sp^3$ bonded atoms.
