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 <cha...@uw.edu> | **P*: +1 (206) 383-3514 On Wed, May 20, 2020 at 5:50 AM M.J. Hutcheon <mjh...@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 > <cha...@uw.edu> | **P*: +1 (206) 383-3514 > > On Wed, May 20, 2020 at 4:43 AM M.J. Hutcheon <mjh...@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 > <cha...@uw.edu> | **P*: +1 (206) 383-3514 > > On Wed, May 20, 2020 at 4:01 AM M.J. Hutcheon <mjh...@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 > <cha...@uw.edu> | **P*: +1 (206) 383-3514 > > _______________________________________________ > Quantum ESPRESSO is supported by MaX (www.max-centre.eu/quantum-espresso) > users mailing list users@lists.quantum-espresso.org > https://lists.quantum-espresso.org/mailman/listinfo/users > > > > >
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