[Wien] Volume has changed during running optimization of c/a with constant volume & b/a

[Fecher, Gerhard]

In the scf file you find the volume:

       LATTICE                      = B   
:POT  : POTENTIAL OPTION EX_PBE EC_PBE VX_PBE VC_PBE
:LAT  : LATTICE CONSTANTS= 11.31946 11.79152 16.06778    1.571    1.571    1.571
:VOL  : UNIT CELL VOLUME =    1072.31280

and 11.31946 * 11.79152 * 16.06778 / 2 = 1072.312797243878022456

so what is the problem, beside the rounding  ?

By the way, what you are doing is "searching for a local minimum" but not an "optimization"!

You have to find the absolute (!) energy minimum in the a, b, c space
In yourprocedure it might depend on which "minimum energy search" you use to start, and on the series of seraches.
You can not expect a priori that only one minimum exists in the range
a +- Delta a, b +- Delta b, c +- Delta c 
and yo might end up in a "wrong" local minimum.

Further, your structure has free position parameters, 
therefore, you have to optimize the positions TOGETHER with the lattice parameters,
obviously you did not do that, the positions are equal in both files ?

For completeness:
The "Volume optimization" alone has only a physical meaning for cubic systems 
(or spherical as most of the equations of states are for the pressure-volume relation inside earth, 
I wonder how they would look like if the earth was a disk, ... or maybe it is ;-).

Ciao
Gerhard

DEEP THOUGHT in D. Adams; Hitchhikers Guide to the Galaxy:
"I think the problem, to be quite honest with you,
is that you have never actually known what the question is."

====================================
Dr. Gerhard H. Fecher
Institut of Inorganic and Analytical Chemistry
Johannes Gutenberg - University
55099 Mainz
and
Max Planck Institute for Chemical Physics of Solids
01187 Dresden
________________________________________
Von: Wien [wien-bounces at zeus.theochem.tuwien.ac.at] im Auftrag von shamik chakrabarti [shamik15041981 at gmail.com]
Gesendet: Samstag, 13. Juni 2020 07:14
An: A Mailing list for WIEN2k users
Betreff: Re: [Wien] Volume has changed during running optimization of c/a with constant volume & b/a

Dear All,
              I am sending the LiNiNbO4_ca_initial.struct file which is obtained after volume optimization with a:b:c =constant. Also I am sending  LiNiNbO4_ba_initial.struct which is obtained after c/a optimization with constant volume and b/a. Both the structures are showing same unit cell volume as ~ 2144 bohr^3 as I have obtained in my calculation where as the printed volume in the w2web page obtained after c/a optimization is  2058.7690 bohr^3 which is different than the actual optimized volume we have obtained. Please comment on this.

with regards,


On Sat, 13 Jun 2020 at 02:03, Tomas Kana <kana at seznam.cz<mailto:kana at seznam.cz>> wrote:

Zpět
[https://i.im.cz/private-avatar/6be7aba869b05ecd061bd77ff52f23ec0d7890f91360a35dc19cfad48b41ad7080ea62f5311a154a/96]Dnes 22:15

Tomas Kana
Komu:
wien at zeus.theochem.tuwien.ac.at<mailto:wien at zeus.theochem.tuwien.ac.at>
Re: [Wien] Volume has changed during running optimization of c/a with constant volume & b/a
Dear Shamik, If I understand you correctly then I suggest to visualise better your energy-volume curve. Try this (it works for me): 1. Keep c/a and b/a experimental ones and run x optimize option 1 and make an energy-volume curve by modifying he option save_lapw -d ../$i  $i in the file optimize.job that is produced by x optimize. 2. For each separated directory run_lapw and collect all the scf files to one one directory and visualise the resulting energy-volume curve by  eplot -t vol 3. In each separated directory optimize c/a  at constant volume and b/a. Again collect all scf files and visualise both "experimental" and "relaxed" energy volume curve in one plot. You will probably note and understand the difference between the two mimimums. 4. Do the same optimization for b/a at constant volume and c/a and draw the three energy-volume curves together. Their minima could again differ. Repeat 3 and 4 until the resulting e-v curve does not substantially differ from the preceding one. Alternatively you may use the package OrthoOpt from wien2k goodies.
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--
Dr. Shamik Chakrabarti
Research Fellow
Department of Physics
Indian Institute of Technology Patna
Bihta-801103
Patna
Bihar, India