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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