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

[mehrdad zamzamian]

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: mehrdad.zamzamian at gmail.com

On Wed, May 29, 2019 at 9:48 PM Kevin May <kmay at mit.edu> 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
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.quantum-espresso.org/pipermail/users/attachments/20190530/0f119857/attachment.html>