I would add:
1. H should be 0.75 if you passivate As (and 1.25 if you passivate Ga)
otherwise you lack several electrons and this will contribute to surface
states.
(I also hope you have 2 pseudoH per As)
2. Secondly, what surface reconstruction do you use?
Most likely you will have a lot of surface states (maybe even a metallic
surface) because of Mn (and depending on surface reconstruction), so
that your results would be very sensitive to the k-point sampling
(despite it is a huge unit cell, you may need a 25A k-grid cutoff rather
than 12A that you use) and thus a very slow SCF convergence.
So yes, using a 1300K electronic temperature was a right choice.
3. 10 lattice constants in z-direction is way too much (which means
(10-3.25)*5.76A=39A of vacuum) - this is not a problem for strictly
localized orbitals, but for the grid it is still a considerable effort.
~15A of vacuum should be typically enough, especially if you turn on the
SlabDipoleCorrection
4. 600Ry is way too much! This is the main reason of your slow
calculation. 300Ry should be enough.
If you really want a better grid try using GridCellSampling (even the
shortest example from the manual would work fine)
Alex.
