ad 1) If I understand correctly, it is because the basis set would change in every step, so the wavefunction from the preceding step couldn’t be used as a starting point for the scf, unless the bands were projected onto the new PW basis. Best, Michal Krompiec Merck KGaA
On Sun, 20 Sep 2020 at 17:25, Robert Molt <r.molt.chemical.phys...@gmail.com> wrote: > If I may ask a question to clarify a point within this thread…I have > always had some confusion on exactly why the the # of basis vectors is not > conserved step-to-step. This is an implicit aspect to the question being > asked, if I understand properly. > > 1.) Why is the *number* of plane waves during vc-relax steps constant? > Why is not held to a constant energy cutoff, like in an energy calculation? > > My understanding is that ecutrho and ecutwfc “constrain” the number of > plane waves during a vc-relax calculation; these two variables completely > specify the number of allowed plane waves consistent with the initial unit > cell volume. Is this correct? > > My understanding is that if you wanted a constant energy cutoff, you have > to vary the number of plane waves, since the volume is changing. Why is > this undesirable to do? Is it because comparing different numbers of plane > waves results in different energies, such that no fair comparison can be > made between calculations with different numbers of basis vectors? > > 2.) On this specific question asked by Mohad, I am slightly confused. Is > the essence of the problem that the pressure calculation threshold is too > low OR is it that you should never use the “properties” other than lattice > constants from a vc-relax for the aforementioned reason? In my head, these > are seemingly independent issues in how the calculations are working? > > Dr. Robert Molt Jr. > Indiana University Purdue University > > > On Sep 20, 2020, at 6:03 AM, Lorenzo Monacelli < > lorenzo.monace...@roma1.infn.it> wrote: > > > > > > > > > > If I interpret correctly the question, you are seeing a P = 6.57 > > kbar and not 7 as in the pw.x input press keyword because, when qe > > relax the unit cell, it constrains the basis defined with ecutwfc > > and ecutrho. This basis, as it is formed by plane waves with > > periodic boundaries, depends on the cell size. So the code will > > relax reaching 7 kbar (within the accuracy you chose in the input, > > usually 0.1 kbar), then it will run a new calculation in the final > > cell recomputing the basis with the new cell. So the value of the > > pressure will be slightly different. > > To be consistent, you should use 6.57 kbar in your plots, as it > > is the final pressure with the cutoffs you used for your > > calculation, and does not depend on the starting cell. If you want > > exactly 7 kbar, you can start a new vc-relax using the final > > structure you have at 6.57 kbar as starting point, and it will get > > to a much closer value. > > Bests, > > Lorenzo > > > > > Il 20/09/20 10:54, Stefano Baroni ha > > scritto: > > > > > > > > > What is difference between the frequencies computed at: i) P=0; > > ii) P=6.57 kbar; iii) 7 kbar? The answer to my question will > > imply that to yours. SB > > > > > > > > > — > > > Stefano Baroni - SISSA, Trieste - http://stefano.baroni.me > , stefanobaroni (Skype) > > > > > > If the prediction that an airplane can stay up depends > > on the difference between Riemann and Lebesgue > > integration, I don’t want to fly in it [Richard W. > > Hammings] > > > > > > > > > > > > > > > > > > > > On 20 Sep 2020, at 10:42, Mohad Abbasnejad > > <mohaddeseh.abbasne...@gmail.com> > > wrote: > > > > > > > > > > > > Hello Dear QE users > > > > > I am studying the effect of pressure on > > the frequencies. > > > In the experimental article, the > > pressure on the solid has been reported to be 7 > > kbar. > > > > > > > > When I put this pressure (7 kbar) on my > > structure and let it relax, the final pressure is > > calculated to be 6.57 kbar as the following. > > Therefore it is not exactly 7 kbar. > > > > > > > > > > Computing stress (Cartesian axis) and > > pressure > > > > > > total stress (Ry/bohr**3) > > (kbar) P= 6.57 > > > 0.00004467 0.00000000 -0.00000000 > > 6.57 0.00 -0.00 > > > 0.00000000 0.00004467 -0.00000000 > > 0.00 6.57 -0.00 > > > -0.00000000 -0.00000000 0.00004467 > > -0.00 -0.00 6.57 > > > > > > > > > > > > > MY QUESTION: Which pressure should I > > report on my paper; 7 kbar or 6.57 kbar? > > > > > > > > > > > > > Any comments would be appreciated. > > > > > > > > Best regards > > > > > Mohaddeseh > > > > > > > > > > > -- > > > --------------------------------------------------------- > > > Mohaddeseh Abbasnejad, > > > Assistant Professor of Physics, > > > Faculty of Physics, > > > Shahid Bahonar University of Kerman, > > > Kerman, Iran > > > P.O. Box 76169-133 > > > Tel: +98 34 31322199 > > > Fax: +98 34 33257434 > > > Cellphone: +98 917 731 7514 > > > E-Mail: mohaddeseh.abbasne...@gmail.com > > > Website: academicstaff.uk.ac.ir/moabbasnejad > > > > > --------------------------------------------------------- > > > > > > > > > > > > _______________________________________________ > > > 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 > > > > > > > > > > > > > > > > > _______________________________________________ > > 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 > > > > > > > > > > _______________________________________________ > 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 > > > _______________________________________________ > > 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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