[QE-users] Difference between Geometry Optimization and a SCF calculation

Wed May 20 14:51:49 CEST 2020

Thank you for the clarification.

Regards,

*Chaman Gupta*
Graduate Research Student, *Novosselov Research Group
<http://depts.washington.edu/nrglab/>, UW*
Graduate Research Student, *Pauzauskie Research Group
<https://depts.washington.edu/pzlab/wordpress/>, UW*
University of Washington Seattle, *M.S. in Mechanical Engineering*
IIT Kharagpur, *B.Tech in Metallurgical and Materials Engineering*
*Linked In <https://www.linkedin.com/in/professorchamangupta> | Email
<chaman at uw.edu> | **P*: +1 (206) 383-3514

On Wed, May 20, 2020 at 5:50 AM M.J. Hutcheon <mjh261 at cam.ac.uk> wrote:

> Dear Chaman,
>
> When I say final geometry, I mean a,b,c,alpha,beta,gamma and all of the
> fractional coordinates of the atoms: enough to specify the structure in
> it's entirety. These can all change with pressure, as well as the unit cell
> shrinking, atoms sometimes move around when the crystal is pressurised.
>
> Or is it all accounted for when I just change the cell geometry (a,b,c and
> alpha,beta,gamma) to calculate the new band structure at a different
> pressure?
>
>
> This is right; you do a geometry optimisation to determine a,b,c
> alpha,beta,gamma and the fractional atom coordinates at the new pressure.
> The geometry is then all you need to determine the bandstructure.
>
> Best,
>
> Michael
>
>
> On 2020-05-20 13:05, Chaman Gupta wrote:
>
> Great explanation. That clears up so many of my questions, and the
> in-general how to approach my problem.
> When you say feed the final geometry, you are referring to the a,b,c and
> alpha,beta,gamma values right?
>
> A fundamental question:
>
> When I say that I have a structure and I apply external pressure to it,
> then I wanna see how the band structure changes. So the band structure is
> basically the electronic energy levels (valence and conduction). Now apart
> from the pressure changing my cell geometry, shouldn't there be an effect
> of this external pressure on these energies? Or is it all accounted for
> when I just change the cell geometry (a,b,c and alpha,beta,gamma) to
> calculate the new band structure at a different pressure?
>
> Regards,
>
> *Chaman Gupta*
> Graduate Research Student, *Novosselov Research Group
> <http://depts.washington.edu/nrglab/>, UW*
> Graduate Research Student, *Pauzauskie Research Group
> <https://depts.washington.edu/pzlab/wordpress/>, UW*
> University of Washington Seattle, *M.S. in Mechanical Engineering*
> IIT Kharagpur, *B.Tech in Metallurgical and Materials Engineering*
> *Linked In <https://www.linkedin.com/in/professorchamangupta> | Email
> <chaman at uw.edu> | **P*: +1 (206) 383-3514
>
> On Wed, May 20, 2020 at 4:43 AM M.J. Hutcheon <mjh261 at cam.ac.uk> wrote:
>
> Dear Chaman,
>
> Firstly, the pressures in QE are in kbar, not GPa (I don't know if this is
> different in the GUI version), see
> https://www.quantum-espresso.org/Doc/INPUT_PW.html. Because the cell
> changes shrinks (and possibly the atoms move) as you increase the pressure,
> you should do a geometry optimisation at each new pressure, before
> calculating anything. The final geometry at each pressure can then be fed
> into further calculations, which now no longer need to know about the
> external pressure, as they only depend on the geometry (this is why the scf
> calculation will not accept a pressure). Anything that QE can calculate can
> be calculated as a function of pressure by this method.
>
> Best,
>
> Michael
>
> On 2020-05-20 12:22, Chaman Gupta wrote:
>
> Thanks a lot Michael for the explanation. Diving a bit more into my
> specific situation, here's my problem statement:
>
> I have a 'Si simple hexagonal phase (Phase V)' which is stable in the
> pressure range of 17 to 30 GPa (approx.). I want to generate the band
> structure for this Si at different Pressure values.
> *Note: My cell dimensions will vary with change in Pressure*
>
> 1. I have the cif file for this phase. Should I in this case run a
> geometry optimization and look at the HOMO LUMO energy difference to
> compute the band gap,
>      or, Should I run a scf calculation and look at the HOMO LUMO energy
> difference to compute the band gap.
>
> 2. When I am running a 'scf' calculation, and I try to enter (P = 19.5
> GPa):
>     &CELL
>     cell_dofree    = "all"
>     cell_dynamics  = "bfgs"
>     press          =  1.95000e+02
>     press_conv_thr =  5.00000e-01
>
> The BURAI does not take in this command when I hit update. Whilst, if I
> enter the same command while doing a geometry optimization it does update
> my input file.
> a. Can this command only be included while doing a geometry optimization
> calculation ('relax' or 'vc-relax')?
> b. Is there any way to include external pressure while doing the scf
> calculation?
>
> 3. Is it even possible to study (generate) band structures at different
> external pressures using QE?
>
> Thanks again for the help.
>
> Regards,
>
> *Chaman Gupta*
> Graduate Research Student, *Novosselov Research Group
> <http://depts.washington.edu/nrglab/>, UW*
> Graduate Research Student, *Pauzauskie Research Group
> <https://depts.washington.edu/pzlab/wordpress/>, UW*
> University of Washington Seattle, *M.S. in Mechanical Engineering*
> IIT Kharagpur, *B.Tech in Metallurgical and Materials Engineering*
> *Linked In <https://www.linkedin.com/in/professorchamangupta> | Email
> <chaman at uw.edu> | **P*: +1 (206) 383-3514
>
> On Wed, May 20, 2020 at 4:01 AM M.J. Hutcheon <mjh261 at cam.ac.uk> wrote:
>
> Dear Chaman,
>
> Essentially, a geometry optimisation moves the atoms around to reduce the
> energy of the system. The energy of the system is calculated using an scf
> calculation. The geometry optimisation is therefore a bunch of scf
> calculations strung together, with a succession of atomic positions
> approaching the minimum-energy positions.
>
> You should carry out a geometry optimisation if you suspect that your
> atoms are not in the ground state for your given parameters.
>
> Best,
>
> Michael Hutcheon
>
> TCM group, Cambridge University
>
>
> On 2020-05-20 11:37, Chaman Gupta wrote:
>
> Hey everyone,
>
> Might be a very naive question, but I have been very confused with this
> recently.
>
> I have a 'Si - simple hexagonal' cif file. I want to see the band
> structure and/or the band gap (HOMO - LUMO).
> 1. I ran a 'scf' calculation on BURAI (a GUI for QE). It generated an
> output file which gave me the Total energy and the energy of HOMO and LUMO.
> 2. Now separately, I ran a 'geometry optimization' calculation (both
> 'relax' and 'vc-relax': thus 2 different calculations), again on BURAI, and
> I got an output file, which gives me Total energy and energy of HOMO and
> LUMO.
>
> *I specified nbnd = 8 (no. of bands) for both the above mentioned
> calulations*
>
> I am confused as in which one is correct or more reliable. And also when
> should we use which calculation?
>
> Help is really appreciated.
>
> Regards,
>
> *Chaman Gupta*
> Graduate Research Student, *Novosselov Research Group
> <http://depts.washington.edu/nrglab/>, UW*
> Graduate Research Student, *Pauzauskie Research Group
> <https://depts.washington.edu/pzlab/wordpress/>, UW*
> University of Washington Seattle, *M.S. in Mechanical Engineering*
> IIT Kharagpur, *B.Tech in Metallurgical and Materials Engineering*
> *Linked In <https://www.linkedin.com/in/professorchamangupta> | Email
> <chaman at uw.edu> | **P*: +1 (206) 383-3514
>
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