# [Pw_forum] stress or total energy

Nichols A. Romero naromero at gmail.com
Fri Dec 29 16:15:10 CET 2006

```Javad,

It is unlikely that you can converge the stress more than a couple of
kilobar. (It is calculated in a complete different way than in classical
MD.) As hinted at in the previous e-mail, the pressure will converge slower
than the energy within the PW pseudopotential scheme. In other words, if the
calculation needs only 40 Ry to converge the energy well, don't be surprised
if you have to go 60 - 80 Ry to converge you stress tensor.

You can also calculate the pressure (roughly) as
P = -dE/dV

where E and V are the total energy and volume. You would need to work out
the unit conversion to kbar.

It is good to look back in the literature to papers that discuss this matter
in more detail:
Phys. Rev. B 32, 3780 (1985).
Phys. Rev. B 52, 8160 (1995).

On 12/28/06, Katalin Gaal-Nagy <katalin.gaal-nagy at physik.uni-regensburg.de>
wrote:
>
>
> one reason for the discrepancy could be that you are not converged enough
> with respect to the number of plane waves, since the stress is very
> sensitive to that quantity. If you have calculated also the stress during
> your convergence tests with respect to the ecut, you can estimate the
> error in the stress.
> Of course one expects that the stress vanishes at the minimum of the total
> energy.
>
> Another reason coul be that you are using the experimental lattice
> constant, which could yield a energy which is not at the minimum of the
> total energy due to the over/underestimation of the lattice constant using
> LDA/GGA. Thus, I suggest to relax also the lattice constant together with
> the c/a in order to obtain the values at the energy minimum of your
> calculation.
>
> However, 10 kbar is a rather small value which could be somehow
> acceptable.
>
> But maybe someone else could give you further hints.
>
> Best wishes,
> Katalin
>
> On Thu, 28 Dec 2006, Javad hashemifar wrote:
>
> > Dear PWscf users
> >
> > I am trying to relax the c lattice parameter of a tetragonal supercell
> > that its a lattice parameter is fixed to some experimental value.
> > Therefore I am calculating this supercell at different values of c
> > (celldm(3)) and looking at the total energy and stress.
> > My feeling is that these two parameter should have consistent behavior
> > and both could be used to find the optimized c. I mean I expect that
> > when total energy is minimized, then the stress should vanish but it
> > is not the case for my calculation and I observe that when the total
> > energy is minimum, there is considerable value of stress (~ -10 kbar)
> > in the system.
> > I would appreciate if some body explain what is the problem and
> > if it is really true that stress and total energy have not the same
> > behavior, which one should be used to find out relaxed lattice
> > parameters. I expect that total energy is the main criteria for
> > relaxation of the lattice parameters ??
> >
> > Best regards
> >
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--
Nichols A. Romero, Ph.D.
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