[Pw_forum] new bfgs: strange behavior doing vc-relax
Eduardo Ariel Menendez Proupin
eariel99 at gmail.com
Wed Apr 20 16:02:41 CEST 2011
Thanks for your tests. Your experiments looks nice to be included in a QE
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.
Departamento de Fisica
Facultad de Ciencias
Universidad de Chile
---------- Mensaje reenviado ----------
From: "Максим Попов" <max.n.popov at gmail.com>
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
thank you very much for expanded answer and sharing the practical tricks.
I've done some computational experiments on bulk Si (cubic conventional
Here is the result (V is volume of initial unit cell, and V0 is equilibrium
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) -
G cutoff = 837.7995 ( 97137 G-vectors) FFT grid: ( 60, 60, 60) -
! 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) -
G cutoff = 775.1830 ( 97137 G-vectors) FFT grid: ( 60, 60, 60) -
! 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
Materials center Leoben (MCL), Leoben, Austria.
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