[Wien] Why unit cell energy is same on increasing pressure ? and Band gap problem with pressure.

[Fecher, Gerhard]

Your lowest energy is at 10GPa,
But the energy from the Birch Murnagham fit should be lowest at 0GPa
Or was there some typo in the table, or did you use different functionals to compare the energies
Note that you should not compare total enegies from mBJ and/or LDA+U with different size of U

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
________________________________________
Von: wien-bounces at zeus.theochem.tuwien.ac.at [wien-bounces at zeus.theochem.tuwien.ac.at]" im Auftrag von "Zaid [raheskoon at gmail.com]
Gesendet: Freitag, 21. Juni 2013 16:46
An: A Mailing list for WIEN2k users
Betreff: Re: [Wien] Why unit cell energy is same on increasing pressure ? and Band gap problem with pressure.

Thank you Sir. Kindly have a look at the collected data below so that you can understand my problem. This data is for a AB2C4 type spinel compound. Would you explain this unit cell energy trend ? Also as you can see band gap starts increasing slightly with the pressure  after a certain limit.
In volume optimization you get optimized volume. If you put that volume in Murnaghan equation of state with pressure (e.g, 10GPa) then you will get the repective new volume at 10GPa from which you can calculate respective lattice constant at 10 GPa (which can be used in optimization process to get structural data at 10 GPa). I hope this is correct way. May you carefully analyze the data to find where is the actual problem.


Pressure

GPa


V (a.u)3 with GGA


U(Internal free anion parameter)

with GGA


E0

(Ry)


Eg with mBJ-GGA


Eg with GGA+U




















0


2099.381


0.2544


-60638.684


2.733


1.685


10


1885.611


0.2529


-60638.686


2.696


1.835


20


1752.858


0.2521


-60638.683


2.646


1.813


30


1658.429


0.2515


-60638.678


2.598


1.789



On Fri, Jun 21, 2013 at 5:37 PM, Stefaan Cottenier <Stefaan.Cottenier at ugent.be<mailto:Stefaan.Cottenier at ugent.be>> wrote:

I am studying pressure dependent study for the first time. On increasing
pressure from 0 to 30 GPa with a step of 10GPa, I got almost same unit
cell enrergy for 0, 10, 20, 30 GPa. Why it is same ? May anyone explain

What is your definition of 'almost'? In that pressure range, total energy will change (increase) by something as 5 mRy/atom. That may look small with respect to a total energy of several thousands of Ry, indeed.

Moreover, how do you 'apply pressure with steps of 10 GPa' in wien2k? Reducing the lattice parameters is the only thing you can do. Are you sure you aren't looking at 4 identical calculations?

Stefaan

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