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

Wed May 20 14:08:05 CEST 2020

Also, the BURAI GUI gives the option to enter in either kbar or GPa.

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 5:05 AM Chaman Gupta <chaman at uw.edu> 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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