[QE-users] higher total energy in the last stage of vc-relax

[Kevin May]

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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