Dear Eduardo, thank you very much for expanded answer and sharing the practical tricks. I've done some computational experiments on bulk Si (cubic conventional cell) vc-relaxation.
Here is the result (V is volume of initial unit cell, and V0 is equilibrium volume): 1) starting from V > V0, i.e. 1/V < 1/V0 -> more G-vectors for vc-relax: G cutoff = 837.7995 ( 101505 G-vectors) FFT grid: ( 60, 60, 60) - vc-relax G cutoff = 837.7995 ( 97137 G-vectors) FFT grid: ( 60, 60, 60) - post-scf ! total energy = -372.89634728 Ry - the last energy in the course of vc-relax ! total energy = -372.89587589 Ry - post-scf energy NB1: # of G-vectors (vc-relax) > # G-vectors(post-scf), and E(the last point vc-relax) < E(post-scf). 1) starting from V < V0, i.e. 1/V > 1/V0 -> more G-vectors for post-scf: G cutoff = 775.1830 ( 90447 G-vectors) FFT grid: ( 60, 60, 60) - vc-relax G cutoff = 775.1830 ( 97137 G-vectors) FFT grid: ( 60, 60, 60) - post-scf ! total energy = -372.89498529 Ry - the last energy in the course of vc-relax ! total energy = -372.89587142 Ry - post-scf energy NB2: # of G-vectors(vc-relax) < # G-vectors(post-scf), and E(the last point vc-relax) > E(post-scf). Comparing these two experiments, one can make a preliminary conclusion: the more G-vectors, the lower the total Energy, provided all other parameters to be fixed. This is easy to understand: plane-wave basis set is complete, that means 2 things (when dealing with truncated bases): 1) E(N+M) < E(N), where N,M - number of plane waves(G-vectors); 2) lim N->infinity of [ E(N+M)-E(N)] = 0. Now it seems to be more clear for me :) Correct me if I'm wrong somewhere. -- Best regards, Max Popov Ph.D. student Materials center Leoben (MCL), Leoben, Austria. 2011/4/19 Eduardo Ariel Menendez Proupin <eariel99 at gmail.com> > >Dear Dr. Giannozzi, > >thank you for the answer! I could find it myself looking in the output > file a bit >more carefully... > >One thing, which is somehow contrary to my expectations, is that the final > scf >energy is higher > >than the last one from vc-relax. Could you, please, elaborate a bit on the > >matter? > > Dear Maxim, > > 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, > 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. > 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. > > The following link may help > > > http://www.quantum-espresso.org/wiki/index.php/Methodological_Background#Stress > > Best regards > > -- > > > Eduardo Menendez > Departamento de Fisica > Facultad de Ciencias > Universidad de Chile > Phone: (56)(2)9787439 > URL: http://fisica.ciencias.uchile.cl/~emenendez > > _______________________________________________ > Pw_forum mailing list > Pw_forum at pwscf.org > http://www.democritos.it/mailman/listinfo/pw_forum > > -------------- next part -------------- An HTML attachment was scrubbed... URL: http://www.democritos.it/pipermail/pw_forum/attachments/20110420/568a61eb/attachment-0001.htm
