[Pw_forum] energy/force/stress obtained from scf versus vc-relax

xirainbow nkxirainbow at gmail.com
Sat May 28 04:14:15 CEST 2011

Dear Amin:

On Sat, May 28, 2011 at 2:26 AM, Amin Torabi <amin.torabi at gmail.com> wrote:

> Dear all,
> I did a vc-relax calculation. Then, I used the cell_parameters and
> atomic_positions obtained at the end of vc-relax to do an scf calculation. I
> didn't change any other settings, like k-point or ecutoff.
> However, the total energy, the total force and the total stress obtained
> from the scf calculation are different form the values obtained at the end
> of the vc-relax calculation. (The vc-relax values are lower.)
>  Why is it so?
You are right.
When you do vc-relax, the G points contained in wave-function does not
change with the cell parameters(That means the energy cutoff changes when
cell parameters change during vc-relax). When you do scf after vc-relax, the
cell parameters of scf is different from that of vc-relax.
Therefore, the G points contained in wave-function of scf differs from that
of vc-relax.
Usually, you should do the second vc-relax  if the cell parameters change a
lot. After the second vc-relax, the force/stress of scf will be close to
that of second vc-relax.

> Thanks is advance.
> Amin
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Hui Wang
School of physics, Fudan University, Shanghai, China
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