*Meshcutoff= 500 Ry is efficient to get accurate calculations. What do you mean "accurate"?*
I mean that I get for bulk calculations for GGA correct lattice constant, bulk modulus. For 500 Ry I get zero forces on atoms for bulk GaAs, whereas for 300 Ry I still have some not vanishing values. * So the question is, what exactly you are trying to study and how you model it (based on what you prepare your starting geometry)?* I want to study a different configuration of one pair of Mn ions on the surface of GaAs. I want to see the difference in the energy , electronic structure, I need to have very accurate calculations , forces should be very small lake 0.002 eV/ A., DM.Tolerance 10-5, is it possible to reach such a calculations on a slab? Magda 2010/7/26 Alexander Vozny <[email protected]> > 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). >
