[Pw_forum] cohesive energy of tungsten

[Gabriele Sclauzero]

Dear Mohnish

Il giorno 06/dic/2010, alle ore 07.51, mohnish pandey ha scritto:

> Dear Vicky!
>                    Check the convergence of energy of isolated atoms also. Normally convergence threshold for energy for isolated species is higher than the crystal system.

I'm sorry, but I hope you'll let me disagree on this point. What makes you say this?

> Check for different values of ecut and use the converged value.

One usually uses the same ecutwfc value for both bulk and atomic calculations, in order to take advantage of some error cancellation. Indeed, if you plot the difference between the bulk and atomic total energies as a function of ecutwfc, you should see that it converges much faster than any of the two energies taken alone.

> And next thing is you are doing calculations for bulk system so all the degrees of freedom should be relaxed for geometry optimization so instead of flag "relax" use "vc-relax".. The former one will converge the forces but not the stress and latter one converges the stress also...

I agree that the "vc-relax" option is useful, but maybe not so much in this case. I think that bulk W is bcc, hence it can be studied with one atom per cell and there are no internal parameters to relax. You can simply scan the total energy as a function of the lattice parameter to get the equilibrium value, as Vicky already did.

There are many other issues that come in, especially in the atomic calculation (see recent discussions on the forum). Even when you get a converged DFT value, in some cases this might be considerably far from the experimental value.

Best regards,

GS

> On Sun, Dec 5, 2010 at 11:53 AM, vicky singh <kirtinandan07 at gmail.com> wrote:
> Hi
> 
> I am trying to calculate the cohesive energy for tungsten. the code is attached. I am able to get the lattice parameter of 3.19 ang while experimental is 3.16 angs. Then i calculated the cohesive energy of w by calculating the energy of isolated w atom by changing  lattice parameter up to 25. I got the cohesive energy of 11.1 ev/atom while reported value is 8.9 ev/atom. This is way too high. can anybody help me. I have varied the K point as well as The E_cut but no advantage.
> 
> vicky singh
> research student
> Bangalore 
> 
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> Regards,
> MOHNISH,
> -----------------------------------------------------------------
> Mohnish Pandey
> Y6927262,5th Year dual degree student,
> Department of Chemical Engineering,
> IIT KANPUR, UP, INDIA
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§ Gabriele Sclauzero, EPFL SB ITP CSEA
   PH H2 462, Station 3, CH-1015 Lausanne

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