<div dir="ltr">The relax option does NOT optimize the lattice constants. (useful when you want to relax impurities of molecules)<div><br></div><div>The vc-relax option optimize the lattice constants. (useful only you have crystalline system)</div>
<div><br></div><div>Now it depends what you want to do: </div><div><br></div><div>Do you have just a crystal? I should say vc-relax is enough. </div><div><br></div><div>Do you have a slab with a molecule attached into the surface? I would first vc-relax the surface alone; I would insert the molecule, then I would optimize the geometry using relax option.<br>
</div><div><br></div><div>If I had just a single molecule, I would create a very large box and just relax the structure.</div><div><br></div><div>In order to be sure that your simulation is ok you must always check the convergence parameters: total energy, k-point mesh, DOS, forces, etc... all the time.</div>
<div>Also, keep in mind the convergence depends on the property you want to investigate.</div><div><br></div><div>Is there anything in particular that you have an issue?<br></div><div><br></div><div>Regards,<br></div><div>
<br></div><div>Filipe</div><div><br></div><div><br></div><div><br></div></div><div class="gmail_extra"><br><br><div class="gmail_quote">On Fri, Apr 25, 2014 at 6:14 PM, Sohail Ahmad <span dir="ltr"><<a href="mailto:sohailphysics@yahoo.co.in" target="_blank">sohailphysics@yahoo.co.in</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div><div style="font-size:14pt;font-family:HelveticaNeue,Helvetica Neue,Helvetica,Arial,Lucida Grande,Sans-Serif"><div>
<span>I am using QE 5.0.2 and I have a very basic question.</span></div><div style="font-style:normal;font-size:18.6667px;background-color:transparent;font-family:HelveticaNeue,Helvetica Neue,Helvetica,Arial,Lucida Grande,Sans-Serif">
<span>To optimise the structure of a material, i started with known experimental lattice constants and calculated energy vs lattice to know the optimised lattice constants. </span></div><div style="font-style:normal;font-size:18.6667px;background-color:transparent;font-family:HelveticaNeue,Helvetica Neue,Helvetica,Arial,Lucida Grande,Sans-Serif">
<span><br></span></div><div style="font-style:normal;font-size:18.6667px;background-color:transparent;font-family:HelveticaNeue,Helvetica Neue,Helvetica,Arial,Lucida Grande,Sans-Serif"><span>Now i wish to apply the strain <br>
</span></div><div style="font-style:normal;font-size:18.6667px;background-color:transparent;font-family:HelveticaNeue,Helvetica Neue,Helvetica,Arial,Lucida Grande,Sans-Serif"><span>so i increased lattice constant 'a' by 2% (let us say)</span></div>
<div style="font-style:normal;font-size:18.6667px;background-color:transparent;font-family:HelveticaNeue,Helvetica Neue,Helvetica,Arial,Lucida Grande,Sans-Serif"><span><br></span></div><div style="font-style:normal;font-size:18.6667px;background-color:transparent;font-family:HelveticaNeue,Helvetica Neue,Helvetica,Arial,Lucida Grande,Sans-Serif">
<span>now i need to relax (i believe) to get optimised coordinates before proceeding further and wish to know force and stress as well.<br></span></div><div style="font-style:normal;font-size:18.6667px;background-color:transparent;font-family:HelveticaNeue,Helvetica Neue,Helvetica,Arial,Lucida Grande,Sans-Serif">
<br><span></span></div><div style="font-style:normal;font-size:18.6667px;background-color:transparent;font-family:HelveticaNeue,Helvetica Neue,Helvetica,Arial,Lucida Grande,Sans-Serif"><span>So my question is i should consider it as 'relax' or "vc relax"</span></div>
<div style="font-style:normal;font-size:18.6667px;background-color:transparent;font-family:HelveticaNeue,Helvetica Neue,Helvetica,Arial,Lucida Grande,Sans-Serif"><span>if relax then following should work but unfortunately no<br>
</span></div><div style="font-style:normal;font-size:18.6667px;background-color:transparent;font-family:HelveticaNeue,Helvetica Neue,Helvetica,Arial,Lucida Grande,Sans-Serif"><span>tprnfor =
.TRUE.</span></div><div style="font-style:normal;font-size:18.6667px;background-color:transparent;font-family:HelveticaNeue,Helvetica Neue,Helvetica,Arial,Lucida Grande,Sans-Serif"><span>tstress = .TRUE.</span></div><div style="font-style:normal;font-size:18.6667px;background-color:transparent;font-family:HelveticaNeue,Helvetica Neue,Helvetica,Arial,Lucida Grande,Sans-Serif">
<span>ion_dynamics = 'bfgs'<br></span></div><div style="font-style:normal;font-size:18.6667px;background-color:transparent;font-family:HelveticaNeue,Helvetica Neue,Helvetica,Arial,Lucida Grande,Sans-Serif">If i donot consider force and stress, then it gives me final coordinates</div>
<div style="font-style:normal;font-size:18.6667px;background-color:transparent;font-family:HelveticaNeue,Helvetica Neue,Helvetica,Arial,Lucida Grande,Sans-Serif"><br></div><div style="font-style:normal;font-size:18.6667px;background-color:transparent;font-family:HelveticaNeue,Helvetica Neue,Helvetica,Arial,Lucida Grande,Sans-Serif">
Is the above step true in this particular set of problems of strain application or should i use variable cell.</div><div style="font-style:normal;font-size:18.6667px;background-color:transparent;font-family:HelveticaNeue,Helvetica Neue,Helvetica,Arial,Lucida Grande,Sans-Serif">
i am little confused.<br><span></span></div><div style="font-style:normal;font-size:18.6667px;background-color:transparent;font-family:HelveticaNeue,Helvetica Neue,Helvetica,Arial,Lucida Grande,Sans-Serif"><span>So<span class="HOEnZb"><font color="#888888"><br>
</font></span></span></div><span class="HOEnZb"><font color="#888888"><div> </div><div>--------------------------------------------------------------------- <br><br><b><font color="#0000ff" size="4">Sohail Ahmad</font></b></div>
<font color="#0000ff">King Khalid
University</font><div><font color="#0000ff">Abha, Saudi Arabia</font></div><div><font color="#0000ff"><br></font>--------------------------------------------------------------------<br></div></font></span></div></div><br>
_______________________________________________<br>
Pw_forum mailing list<br>
<a href="mailto:Pw_forum@pwscf.org">Pw_forum@pwscf.org</a><br>
<a href="http://pwscf.org/mailman/listinfo/pw_forum" target="_blank">http://pwscf.org/mailman/listinfo/pw_forum</a><br></blockquote></div><br><br clear="all"><div><br></div>-- <br><div dir="ltr">_________________________________________<br>
Filipe Camargo Dalmatti Alves Lima<br>PhD Student<br>University of São Paulo, Physics Institute, Materials Physics Department, Nanomol Group, Brazil.<br>Phones: (11) 3091-6881 (USP)<br> (11) 97408-2755 (Vivo)<br>
</div>
</div>