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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="reply-intro">On 2020-05-20 11:37, Chaman Gupta wrote:</p>
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<div class="v1gmail_default">Hey everyone,</div>
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<div class="v1gmail_default">Might be a very naive question, but I have been very confused with this recently.</div>
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<div class="v1gmail_default">I have a 'Si - simple hexagonal' cif file. I want to see the band structure and/or the band gap (HOMO - LUMO).</div>
<div class="v1gmail_default">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 class="v1gmail_default">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 class="v1gmail_default">*I specified nbnd = 8 (no. of bands) for both the above mentioned calulations*</div>
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<div class="v1gmail_default">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 class="v1gmail_default">Help is really appreciated.</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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