# Re: [Wien] How to get lattice parameter as a function of pressure?

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```Can you tell me how to extract lattice parameter as a function of pressure?
I didn't find it in case.eosfit as mentioned here
http://www.mail-archive.com/wien%40zeus.theochem.tuwien.ac.at/msg04276.html
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It would be trivial to do this for a crystal that is entirely determined by the volume (for instance, bcc iron): you calculate energy for a few volumes, do the Birch-Murnaghan fit by eosfit, look for the volume vs. pressure data in case.eosfitb, and convert the volume to lattice parameter.
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It becomes somewhat more complicated when you have internal degrees of freedom (let's still assume a cubic crystal for the time being). Again calculate the total energy for a set of volumes. But for every volume, the internal positions should now be optimized first. Once you have done that, the procedure is the same as before, and you can get the lattice parameters out of the volume.
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Now drop the requirement of a cubic crystal. Take an orthorhombic one (with or without internal degrees of freedom). Again you need the total energy for a set of volumes. But for every volume you need optimize internal parameters and the three lattice parameters (with the restriction that the volume is kept constant). From these optimizations, you will know what the relation between volume and the three different lattice parameters is. From the fitted volume-pressure relation, you can get any of the lattice parameters as a function of pressure.
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Stefaan

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After x eosfit, in the case.eosfit, I can only find three colums, which
I think corresponding to volum, energy and pressure. But no lattice
constant information. I can't find any lattice constant information in
case.outputeos either.

I guess eosfit6 probably can do this job, however, I don't know how to
use it. Before doing x eosfit, I did,  parabolfit_lapw,  but it always
gives me errors. (see blow)

I hope you can give me some idea. Thanks in advance.

1)  do x optimize and create 36 cases.
2)  run optimize.job with "save_lapw -d optimize \${i}" which saves all
my data into directory optimize.
3)  grepline :ENE "optimize/*scf" 1 >> case.analysis
grepline :VOL "optimize/*scf" 1 >> case.analysis
4) eplot_lapw -t vol   This step gives me the plot Energy vs Vol.
5) x eosfit
6) parabolfit_lapw -t 2 -f optimize -scf 'optimize/*.scf'   : This step
gives error like this

ls: No match.
ls: No match.
optimize.ene and optimize.latparam generated
ERROR IN OPENING UNIT:          10
FILENAME:
ZrPSe.ene
STATUS: old          FORM:formatted
OPEN FAILED
0.0u 0.0s 0:00.00 0.0% 0+0k 0+0io 0pf+0w

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--
Stefaan Cottenier
Center for Molecular Modeling (CMM) &
Department of Materials Science and Engineering (DMSE)
Ghent University
Tech Lane Ghent Science Park – Campus A
building 903
BE-9052 Zwijnaarde
Belgium

http://molmod.ugent.be
http://www.ugent.be/ea/dmse/en
email: stefaan.cotten...@ugent.be

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for China: http://i.youku.com/cottenierlectures
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