[Wien] Requesting gentle clarification on OMP_NUM_THREADS, run_lapw, volume optimisation and wien2k update.

[Laurence Marks]

Inlined

--
Professor Laurence Marks (Laurie)
Department of Materials Science and Engineering, Northwestern University
www.numis.northwestern.edu
"Research is to see what everybody else has seen, and to think what nobody
else has thought" Albert Szent-Györgyi

On Thu, Jun 15, 2023, 16:54 Pranjal Nandi <pnandi at ub.edu> wrote:

> Dear Sir/Madam,
>
> Hello.
>
> My WIEN2k version is 21.1
>
> I am fairly new to using WIEN2K and have been going through the guidebook
> as well as running smaller simulations over the last few months. After
> getting some confidence I am now trying to simulatate oxygen vacancies in
> HfO to find the optic properties. I have created a supercell 2x2x2 and
> deleted 3 Oxygen atoms from the struct file. Presently when I run the
> simulation, overall there are 216 atoms.
>
I have some general queries and it would be great if you can kindly provide
> me your feedback.
>
>
>    1. I have 64 CPU (32 x 2) and more than 9 TB space and the system is
>    dedicated entirely to do simulations on DFT. I was thinking to set the
>    OMP_NUM_THREADS at 40 or 50 (even higher) to use the maximum potential of
>    the computer. But in the guidebook, it is recommended that I do not set it
>    higher than 8. May I please the reason for this recommendation?
>
> You will find that while 2 OMP are about 2x as fast as 1, 4 are less so
(perhaps 3.5) and so on. Above 8 is not efficient. You should instead use
k-point parallel, please see the UG, or use mpi.

2. Is there a code to run_lapw continuously till it reaches the convergence
and not start it again after 40 cycles?

You can set the number of iterations to N using "-i N". You can also
continue from where you were using the -NI option.

>
>    1.
>    2. If I do a volume optimization for a single unit cell and then I
>    create a supercell of the optimized structure. Does the supercell remains
>    in optimized volume condition? Or it is recommended that I do a volume
>    optimization for the supercell as well.
>
> This is tricky! If you are simulating a few vacancies in otherwise bulk,
then the surrounding bulk sets the lattice parameters and you should not
change them. If you are trying to simulate reduced HfO with a large number
of vacancies then you should relax the lattice parameters. In both cases
you need to relax the internal positions (-min).


>    1. Further is WIEN2k a reliable platform for volume optimization of
>    HfO as in the guidebook on page 87 it mentions “The accuracy is thus
>    limited to first and second row elements (maybe early 3d elements) and in
>    particular for the 5d series and above, large errors may occur due to the
>    neglect of (scalar-) relativistic corrections.”
>
> The biggest issue to worry about first is the treatment of the 4f
electrons. This requires an on-site hybrid or, not so good, a Hubbard U.
You should read more general DFT background first, this is not simple.

>
>    1.
>    2. Lastly, as it says on the website that it is highly recommended to
>    update the WIEN2K version, I am unable to find the manual where I can
>    follow the instructions to update the version. As I have not done any
>    installation of WIEN2K (my past colleague installed the present version), I
>    am worried that while upgrading I may commit some mistake. A guideline on
>    updating would have been very helpful.
>
> You need to get the password from your colleague first. If you do not have
this you should contact Peter Blaha directly.

>
>
> Looking forward to your kind feedback.
>
>
>
> Thank you in advance.
>
>
>
> With warm regards,
>
> Pranjal
>
>
>
>
>
>
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