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

[Peter Blaha]

I expect that the pressure you listen is an experimental pressure and 
the corresponding volume.
However, in theory you do NOT find the same lattice parameters as in 
exp., thus you must read off the pressure from case.outputeos.

About gap: it is quite natural that the gaps decrease with pressure. 
Most insulators turn into metals if the pressure is high enough.

Am 21.06.2013 16:46, schrieb Zaid:
> 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*
>
> 	
>
> *E_0 *
>
> *(Ry)*
>
> 	
>
> *E_g with mBJ-GGA*
>
> 	
>
> *E_g 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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-- 
Peter Blaha
Inst.Materials Chemistry
TU Vienna
Getreidemarkt 9
A-1060 Vienna
Austria
+43-1-5880115671