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

[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*

*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> 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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