[Pw_forum] equation of state fitting

Sunetra Das sunetra.das.88 at gmail.com
Wed Aug 16 20:52:58 CEST 2017

Thank You all for your kind help and suggestions.

I would further like to clarify a few more things from the suggestions

Dear Dr. Michael Mehl,

Quoting you from your reply, "You must have minimum energy structure for
your crystal structure at that volume.  This means minimizing the energy
with respect to all atomic positions and all lattice parameters: b/a, c/a,
etc., while keeping the volume fixed."

I have to keep the volume of my structure fixed during the structural
optimization? Is it the equilibrium volume obtained from the equation of
fitting, that I have to keep fixed, while varying the other structural
parameters? As you said the equilibrium lattice parameter comes very close
to the experimental value given in the tutorial, but the bulk modulus value
deviates beyond the 10% range in case of all the equations of state,
specially for the Keane equation it deviates hugely while for the 2nd order
Birch equation the deviation is the least. From the output data obtained
after the fittings, the 'E_diff' values (that is the difference between the
calculated total energy data and the fitted data at a given lattice
parameter) are of the order of 10^-4 Ry and in case of the 2nd order Birch
equation all the E_diff values are nil.

Dear Dr. Alberto,
Thank you for the link to the article. I will follow your suggestions and
try again.

Dear Dr. Lorenzo,
Yes sir, the structure obtained is satisfactory and the equilibrium lattice
parameter is also very close to the experimental value.
I have already tested the convergence of the energy with respect to the
ecutwfc value, as was instructed in the tutorial. So I should keep on
increasing the parameter and check every time if the experimental value is
getting reproduced and stick to that ecutwfc value? Please correct me if I
am wrong. I will also try the 2nd method suggested by you.

Thank you again to all of you.
Sunetra Das.

On 16 August 2017 at 23:07, Lorenzo Paulatto <lorenzo.paulatto at impmc.upmc.fr
> wrote:

> On 16/08/17 18:36, Sunetra Das wrote:
> >  those equation fitting match the given experimental value, not even
> > close.
> Dear Sunetra,
> Is your structure satisfactory? Can you reproduce the experimental
> geometry and volume?
> If not, you won't be ablt to get a good bulk modulus.
> If yes, try increasing the ecutwfc quite a bit, or run your calculations
> at constant number of plane waves, which if I remember correctly means
> changing ecutwfc ~ alat^-2
> hth
> --
> Dr. Lorenzo Paulatto
> IdR @ IMPMC -- CNRS & Université Paris 6
> phone: +33 (0)1 442 79822 / skype: paulatz
> www:   http://www-int.impmc.upmc.fr/~paulatto/
> mail:  23-24/423 Boîte courrier 115, 4 place Jussieu 75252 Paris Cédex 05
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