Thanks for your attention to my question Actually, i defined a supercell (2*2*2), used USPP-PBE.I adjusted the cutoff according to what was proposed for it (~70) and for ecut_rho (~400). for scf convergence, i set conv=1e-6, ant at here, i set mixing beta~0.04 (it helped to reach to convergence fast). and also i set press=0 and its threshold to 0.5. I want to calculate the vacancy (lack of Fe) energy. its value must be 1.6 eV but i never give the better than 2.2 eV (The ridiculous thing is that with molecular dynamics I got a very precise amount of 1.6! that of course, its interatomic is more precise than my calculation!). according to my PP in this calculation, it is suggested to use non-polarized spin. i really do not know what parameter can i changed to achieve 1.6 eV. In addition, your food for the brain is some hard to be digested!
Regards Seyed Mehrdad Zamzamian Sharif University of Technology, Tehran, Iran Energy engineering department E-mail: [email protected] On Wed, May 29, 2019 at 9:48 PM Kevin May <[email protected]> wrote: > Hi Mehrdad, > > If you have a large energy difference between the end of the vc-relax > algorithm and the final scf step, you may be using a fairly low plane wave > cutoff energy for your system, though that may be fine depending on what > quantities you are looking at. An important question here is: how are you > choosing your convergence criteria? In my experience stresses (vc-relax) > require a higher cutoff compared to total energy or forces (just ionic > relaxation). > > This all depends of course on what you are trying to get out of your > calculation. In my case I was comparing different magnetic ground states > that could be very close in energy, where very small changes in geometry > can make a difference. For my specific systems I would converge plane wave > cutoff, k-points and convergence threshold with respect to unit cell stress > using a reference calculation with very high cutoff, low threshold (10^-9 > Ry), and dense k-point mesh. > > Just some food for thought. I've definitely seen papers in the literature > where they claim state A has lower energy than state B. I've reproduced > such results using the somewhat lax cutoffs reported, and then found when > you actually do CONVERGED calculations, state B is actually lower in energy > (whoops!). Convergence is important. > > Best, > > Kevin May, PhD > Postdoctoral Associate > Department of Materials Science and Engineering > Massachusetts Institute of Technology >
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