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<p>Dear Chaman,</p>
<p>Firstly, the pressures in QE are in kbar, not GPa (I don't know if this is different in the GUI version), see <a href="https://www.quantum-espresso.org/Doc/INPUT_PW.html">https://www.quantum-espresso.org/Doc/INPUT_PW.html</a>. 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.</p>
<p>Best,</p>
<p>Michael</p>
<p id="reply-intro">On 2020-05-20 12:22, Chaman Gupta wrote:</p>
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<div class="v1gmail_default" style="font-family: trebuchet ms,sans-serif; font-size: small; color: #000000;">Thanks a lot Michael for the explanation. Diving a bit more into my specific situation, here's my problem statement:</div>
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<div class="v1gmail_default" style="font-family: trebuchet ms,sans-serif; font-size: small; color: #000000;">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.</div>
<div class="v1gmail_default" style="font-family: trebuchet ms,sans-serif; font-size: small; color: #000000;">*Note: My cell dimensions will vary with change in Pressure*</div>
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<div class="v1gmail_default" style="font-family: trebuchet ms,sans-serif; font-size: small; color: #000000;">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,</div>
<div class="v1gmail_default" style="font-family: trebuchet ms,sans-serif; font-size: small; color: #000000;"> or, Should I run a scf calculation and look at the HOMO LUMO energy difference to compute the band gap.</div>
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<div class="v1gmail_default" style="font-family: trebuchet ms,sans-serif; font-size: small; color: #000000;">2. When I am running a 'scf' calculation, and I try to enter (P = 19.5 GPa):</div>
<div class="v1gmail_default" style="font-family: trebuchet ms,sans-serif; font-size: small; color: #000000;"> <span style="font-family: Arial,Helvetica,sans-serif; color: #222222;">&CELL</span></div>
cell_dofree = "all"<br /> cell_dynamics = "bfgs"<br /> press = 1.95000e+02<br /> press_conv_thr = 5.00000e-01
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<div class="v1gmail_default" style="font-family: 'trebuchet ms',sans-serif; font-size: small; color: #000000;">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.</div>
<div class="v1gmail_default" style="font-family: 'trebuchet ms',sans-serif; font-size: small; color: #000000;">a. Can this command only be included while doing a geometry optimization calculation ('relax' or 'vc-relax')?</div>
<div class="v1gmail_default" style="font-family: 'trebuchet ms',sans-serif; font-size: small; color: #000000;">b. Is there any way to include external pressure while doing the scf calculation?</div>
<div class="v1gmail_default" style="font-family: 'trebuchet ms',sans-serif; font-size: small; color: #000000;"> </div>
<div class="v1gmail_default" style="font-family: 'trebuchet ms',sans-serif; font-size: small; color: #000000;">3. Is it even possible to study (generate) band structures at different external pressures using QE?</div>
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<div class="v1gmail_default" style="font-family: trebuchet ms,sans-serif; font-size: small; color: #000000;">Thanks again for the help.</div>
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<div dir="ltr"><span style="font-family: trebuchet ms, sans-serif;">Regards,</span>
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<div><strong><span style="font-family: trebuchet ms, sans-serif;">Chaman Gupta</span></strong>
<div><span style="font-family: trebuchet ms, sans-serif;">Graduate Research Student, <em><a href="http://depts.washington.edu/nrglab/" target="_blank" rel="noopener noreferrer">Novosselov Research Group</a>, UW</em><br /></span></div>
<div><span style="font-family: trebuchet ms, sans-serif;">Graduate Research Student, <em><a href="https://depts.washington.edu/pzlab/wordpress/" target="_blank" rel="noopener noreferrer">Pauzauskie Research Group</a>, UW</em></span></div>
<div><span style="font-family: trebuchet ms, sans-serif;">University of Washington Seattle, <em>M.S. in Mechanical Engineering</em></span></div>
<div><span style="font-family: trebuchet ms, sans-serif;">IIT Kharagpur, <em>B.Tech in Metallurgical and Materials Engineering</em></span></div>
<div><span style="font-family: trebuchet ms, sans-serif;"><strong><a href="https://www.linkedin.com/in/professorchamangupta" target="_blank" rel="noopener noreferrer">Linked In</a> | <a href="mailto:chaman@uw.edu" rel="noreferrer">Email</a> | </strong></span><span style="font-family: 'trebuchet ms',sans-serif;"><strong>P</strong>: +1 (206) 383-3514</span></div>
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<div class="v1gmail_attr" dir="ltr">On Wed, May 20, 2020 at 4:01 AM M.J. Hutcheon <<a href="mailto:mjh261@cam.ac.uk" rel="noreferrer">mjh261@cam.ac.uk</a>> wrote:</div>
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<p>Dear Chaman,</p>
<p>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.</p>
<p>You should carry out a geometry optimisation if you suspect that your atoms are not in the ground state for your given parameters.</p>
<p>Best,</p>
<p>Michael Hutcheon</p>
<p>TCM group, Cambridge University</p>
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<p id="v1gmail-m_-3508657660197467182reply-intro">On 2020-05-20 11:37, Chaman Gupta wrote:</p>
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<div>Hey everyone,</div>
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<div>Might be a very naive question, but I have been very confused with this recently.</div>
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<div>I have a 'Si - simple hexagonal' cif file. I want to see the band structure and/or the band gap (HOMO - LUMO).</div>
<div>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.</div>
<div>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.</div>
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<div>*I specified nbnd = 8 (no. of bands) for both the above mentioned calulations*</div>
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<div>I am confused as in which one is correct or more reliable. And also when should we use which calculation?</div>
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<div>Help is really appreciated.</div>
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<div dir="ltr"><span style="font-family: 'trebuchet ms',sans-serif;">Regards,</span>
<div><span style="font-family: 'trebuchet ms',sans-serif;"> </span></div>
<div><strong><span style="font-family: 'trebuchet ms',sans-serif;">Chaman Gupta</span></strong>
<div><span style="font-family: 'trebuchet ms',sans-serif;">Graduate Research Student, <em><a href="http://depts.washington.edu/nrglab/" target="_blank" rel="noopener noreferrer">Novosselov Research Group</a>, UW</em><br /></span></div>
<div><span style="font-family: 'trebuchet ms',sans-serif;">Graduate Research Student, <em><a href="https://depts.washington.edu/pzlab/wordpress/" target="_blank" rel="noopener noreferrer">Pauzauskie Research Group</a>, UW</em></span></div>
<div><span style="font-family: 'trebuchet ms',sans-serif;">University of Washington Seattle, <em>M.S. in Mechanical Engineering</em></span></div>
<div><span style="font-family: 'trebuchet ms',sans-serif;">IIT Kharagpur, <em>B.Tech in Metallurgical and Materials Engineering</em></span></div>
<div><span style="font-family: 'trebuchet ms',sans-serif;"><strong><a href="https://www.linkedin.com/in/professorchamangupta" target="_blank" rel="noopener noreferrer">Linked In</a> | <a href="mailto:chaman@uw.edu" rel="noreferrer">Email</a> | </strong></span><span style="font-family: 'trebuchet ms',sans-serif;"><strong>P</strong>: +1 (206) 383-3514</span></div>
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