# [Pw_forum] new bfgs: strange behavior doing vc-relax

Eduardo Ariel Menendez Proupin eariel99 at gmail.com
Tue Apr 19 12:56:03 CEST 2011

```>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.

http://www.quantum-espresso.org/wiki/index.php/Methodological_Background#Stress

Best regards

--

Eduardo Menendez
Departamento de Fisica