<div><span class="Apple-style-span" style="font-family: arial, sans-serif; font-size: 13px; border-collapse: collapse; ">>Dear Dr. Giannozzi,<br>>thank you for the answer! I could find it myself looking in the output file a bit >more carefully...<br>
>One thing, which is somehow contrary to my expectations, is that the final scf >energy is higher<br>>than the last one from vc-relax. Could you, please, elaborate a bit on the >matter?<br><br>Dear Maxim, <br>
</span></div><div><span class="Apple-style-span" style="font-family: arial, sans-serif; font-size: 13px; border-collapse: collapse; "><br></span></div><div><span class="Apple-style-span" style="font-family: arial, sans-serif; font-size: 13px; border-collapse: collapse; ">I followed this discussion with interest, and thanks to that I learned about the new scf calculation with final G-vectors. Concerning your last question, </span></div>
<div><span class="Apple-style-span" style="font-family: arial, sans-serif; font-size: 13px; border-collapse: collapse; ">the energy is higher because the vc-relaxed energy was optimized for a different basis set, than the final scf calculation (different G-vectors). Hence, the energy of the final scf calculation is is made for a structure that is slightly out of the minimum for the new basis set. Remember than the G-vectors used in a scf calculation are all the reciprocal lattice vectors contained in a sphere that has a radius determined by the cutoff. These vectors are selected at the first step of the vc-relaxation. When the unit cell gets deformed, the G-vectors vary accordingly, and the region that the G-vectors occupy is a deformation from the initial sphere, maybe an elipsoid. When the vc-relax stops, the final scf calculation takes the G-vectors contained inside a sphere. Hence, some of the old G-vectors that were in the border of the deformed sphere may be eliminated, and some that were absent are now included. </span></div>
<div><font class="Apple-style-span" face="arial, sans-serif"><span class="Apple-style-span" style="border-collapse: collapse;">If you had used an (impossible) infinite cutoff, the basis set would be complete in both cases (G-vectors contained in an infinite sphere or in an infinite elipsoid) and there would be no difference. Usually, I repeat the vc-relax procedure starting 'from_scratch' with the last structure (coordinates and lattice vectors) in the new input file, until the vc-relax procedure performs only one step. In this case there is no difference. If it never happens that vc-relax stops at the first step, then I increase the cutoffs. In your case, the energy difference of 0.5 mRy may be small enough and do not need to do that. It depends on the property that you want. E.g., if you are interested in elastic properties, you may need that the minimal energy structure also gives a stress tensor below 0.1 kbar or so. If you cannot get it, increase the cutoff. </span></font></div>
<div><font class="Apple-style-span" face="arial, sans-serif"><span class="Apple-style-span" style="border-collapse: collapse;"><br></span></font></div><div><font class="Apple-style-span" face="arial, sans-serif"><span class="Apple-style-span" style="border-collapse: collapse;">The following link may help</span></font></div>
<br clear="all"><a href="http://www.quantum-espresso.org/wiki/index.php/Methodological_Background#Stress">http://www.quantum-espresso.org/wiki/index.php/Methodological_Background#Stress</a><br><br><div>Best regards</div><div>
<br></div><div>-- <br><div><br></div>
<div><br></div>Eduardo Menendez<br>Departamento de Fisica<br>Facultad de Ciencias<br>Universidad de Chile<br>Phone: (56)(2)9787439<br>URL: <a href="http://fisica.ciencias.uchile.cl/~emenendez" target="_blank">http://fisica.ciencias.uchile.cl/~emenendez</a><br>
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