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

[M.J. Hutcheon]

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 [1], UW_
> 
> Graduate Research Student, _Pauzauskie Research Group [2], UW_ 
> University of Washington Seattle, _M.S. in Mechanical Engineering_ 
> IIT Kharagpur, _B.Tech in Metallurgical and Materials Engineering_ 
> Linked In [3] | Email | 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 [1], UW_
> 
> Graduate Research Student, _Pauzauskie Research Group [2], UW_ 
> University of Washington Seattle, _M.S. in Mechanical Engineering_ 
> IIT Kharagpur, _B.Tech in Metallurgical and Materials Engineering_ 
> Linked In [3] | Email | P: +1 (206) 383-3514 
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Links:
------
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[2] https://depts.washington.edu/pzlab/wordpress/
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