Dear Max, Thanks for your tests. Your experiments looks nice to be included in a QE tutorial. I think your reasoning is correct. Moreover, the energy should always decrease when you enlarge de basis set (adding G-vectors) because the calculation is variational (within a given density functional). I am not sure if the calculation is still variational with ultrasoft pseudopotentials, or even if the Hohenberg-Kohn lemma is valid with non local pseudopotentials. With QE I have always seen the energy to decrease when the cutoffs are increased, but with VASP, I always see an oscillation in the total energy when the cutoff is incremented. I hope one of our professors can clarify this point. Best regards Eduardo
Eduardo Menendez Departamento de Fisica Facultad de Ciencias Universidad de Chile Phone: (56)(2)9787439 URL: http://fisica.ciencias.uchile.cl/~emenendez ---------- Mensaje reenviado ---------- From: "?????? ?????" <[email protected]> To: PWSCF Forum <pw_forum at pwscf.org> Date: Wed, 20 Apr 2011 11:25:46 +0200 Subject: Re: [Pw_forum] new bfgs: strange behavior doing vc-relax 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. -- -------------- next part -------------- An HTML attachment was scrubbed... URL: http://www.democritos.it/pipermail/pw_forum/attachments/20110420/8b9ae36b/attachment.htm
