How to apply a force on the bulk system

[rosen]

it will be usefull to apply the pressure on the bulk system to simulate the high pressure experiment?
is it any way to apply the hydrostatic pressure or a axis compress in Z direction?

[Nordland]

It depends on what you want to study - if you want to see how the band gap for instance behave under pressure,
you could just make two calculations with very close lattice constants and then compare the difference in energy bandgap
to determinethe stress.

However if you want strain or stress analysis it requires some thing and somre sparetime.

[rosen]

Dear Nordland, I am interested in your suggestion, "From the bandgap to determine the stress", I have read some papers, and they just investigate how the bandgap change with the stress, which is what I want to do.

In your suggestion, it seems that we should first construct some structures of the same bulk material, but have slightly difference in the lattice constants to simulate the effect of the pressure, after the internal atomic relaxation and calculate its energy and bandgap, then using the Birch–Murnaghan state of equations to fit the energy and the volume change, we can get the exact pressure value. Then we can have the corresponding bandgap and the pressure relationship.

This is useful for the hydrostatic pressure, what about for the axis pressure, such as compress in Z direction?

[Anders Blom]

I think it's quite straightforward to apply the stress uniaxially by just stretching the unit cell in one direction, and let the relaxation take care of the rest. The easiest way is to convert the structure to a PeriodicAtomConfiguration (PAC) first, so that you have control of the individual unit cell axes, rather than just manipulating the overall lattice constant.

In VNL you can convert a bulk system to a PAC in the Bulk Builder instrument (only released in 2008.10, the latest version, available from the QuantumWise homepage!) with a click of the mouse!

[rosen]

 Anders Blom, Thank you for that suggestion, I will try its new function

[Nordland]

If you are looking into structures under stress/strain, you might want to be interested in trying out
my improved relaxation routines for bulk-crystals , that takes into account the stress and strain of the crystal.
You can find the script and details here (Improved optimizers).

[Nordland]

Hey Rosen.

I found this script that I have made some time ago. It is a module for creating a strain on bulk configuration
in order to model the behavior of the bandgap under strain for GaN.

I have uploaded the functionality here, and the results I got from my studies on GaN.

Perhaps you can use it for you work as well.

[Roc]

Thank you Nordland , great job!

[rosen]

Hey Rosen.

I found this script that I have made some time ago. It is a module for creating a strain on bulk configuration
in order to model the behavior of the bandgap under strain for GaN.

I have uploaded the functionality here, and the results I got from my studies on GaN.

Perhaps you can use it for you work as well.

is it possible to post the script of the GaN example under strain.?

[Nordland]

Hey Rosen.

There are the two files that I have used for generating this plot above.

[rosen]

Hi, Nordland
Thank you for the script, but i still have a question in the last two line of the GaN.py

Code:

bands = calculateEnergyBands(scf,[(0.0,0.0,0.0)])
print '# Strain = %s Band gap = %s'%(ezz, bands.band(8)[0] - bands.band(7)[0])

how do you know that the gap is between bands.band( 8 )[0] - bands.band(7)[0], should we do some bandstruture before to define the position of the VBM (bands.band(7)[0]) and CBM (bands.band( 8 )[0])?

Moderator edit: placed code in (code) to avoid sunglasses (bb code eight) 8 )

[Nordland]

Well I cheated, and perform a calculation prior to this one to determine the number of electrons to make this easy