[QE-users] query about system dependent etot_conv_thr, forc_conv_thr and conv_thr

Dr. K. C. Bhamu kcbhamu85 at gmail.com
Sat May 5 20:55:17 CEST 2018

Just to add some data to my previous post:

The difference in scf energy (in Ry) for my system having 36 atoms is:

ecut vs energy

30 -1.42256
40 -0.0765211
45 -0.00623776
50 -0.00420366
60 -0.00482638
65 -0.00095532

So if I divide energy wrt 45ecut by  36 (number of atoms), it comes out:
-0.0001732 Ry or -0.002356487 eV/atom
For 50 and 60 ecut this difference becomes ~0.00017 Ry or ~0.0016 eV/atom.

For the ecut (the difference between ecut 60 and 65) the  difference is:
-0.000026 Ry/atom or 0.00036eV/atom.

What value of ecut is the perfect choice for my case: 45 or 50 or 60?

I just doing vc relaxation with free xyz.

Kind regards


On Sat, May 5, 2018 at 11:12 PM, Dr. K. C. Bhamu <kcbhamu85 at gmail.com>

> Dear QE Users,
> For simple compounds (binary or ternary), I know the said parameters may
> be fixed tight  (etot_conv_thr, 1e-5,  and forc_conv_thr, 1e-5 and
> conv_thr. 1d-07 or 1d-08 or 1d-09). But for complex systems, if we set them
> high then it would cost much wall time.
> To avoid this, Is there any thumb rule to decide the above parameters to
> be kept fix for complex bulk structures (having a combination of organic
> and inorganic elements), i.e. as the number of atoms increases in the
> system the above parameters can be lowered?
> As far as I know, these parameters have much importance if we see per
> atom. So, if we increase the number of atoms the parameters can be lowered
> (Ex. for hybrid perovskites with 36 atoms in a unit cell, etot_conv_thr,
> 1e-3,  and forc_conv_thr, 1e-3 and conv_thr. 1d-06 ), is it?
> Any comment will be helpful to improve my understanding or in correcting
> me if I am wrong, also will save computational cost.
> Thanks and regards,
> Dr. K. C. Bhamu
> National Postdoctoral Fellow,
> Physical and Materials Chemistry Division
> CSIR-NCL, Pune
> Mob. No.  +91-9975238952
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