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

[Chaman Gupta]

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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