Do you think that I should also optimize the position of Hydroden atoms
or I shoudn't care about it?
I always did, but from my experience this should not matter much. So, in
principle, you can just fix them in some predicted ideal/expected positions.
(don't forget to fix the lowest Ga or As (actually both is even more
correct) in the ideal bulk positions ("ideal" means ideal for your set
of parameters))
I don't use any reconstruction, I just optimize atoms on the surface.
Optimazed surface is for me equal to reconstructed surface, am I right?
No.
Ideal (or correct) reconstruction may even have a different
stoichiometry, so whatever you do (even molecular dynamics at 1000K)
will not help you to reach the correct geometry.
So the question is, what exactly you are trying to study and how you
model it (based on what you prepare your starting geometry)?
But I checked that for bulk calculation, cell containing 64 GaAs atoms,
Meshcutoff= 500 Ry is efficient to get accurate calculations.
What do you mean "accurate"?
Even for bulk GaAs with 2 atoms in unit cell you won't get convergence
even at 800Ry.
This is what Andrei Postnikov always says (search for his older
messages) - mesh cutoff is not a variational parameter (do I say it
correctly?), so you should not check "convergence" for it (in a sense
you do it for k-grid, for example). So mesh cutoff should be only as big
as to provide the correct forces on atoms i.e. get rid of eggbox effect
(try shifting your origin by a small value and see if the forces remain
the same or simply check whether your forces on atoms in ideal bulk
geometry are exactly zero (not sure though that this is equivalent to
the above mentioned requirement)).
GridCellSampling serves exactly for this.
So (for GGA (for GaAs (not sure about Mn))) 300Ry is the maximum that I
would use (even ~200Ry could be in principle enough).
