[Pw_forum] cohesive energy of tungsten

[vicky singh]

sorry for late reply. so what is your suggestion for getting the accurate
value for cohesive energy for W. I also tried another BCC material Ta and
its cohesive energy is also more than 22% higher than the reported value
(kittel). But when i calculated the cohesive energy for FCC Ni and Al the
difference from experimental result is around 6 % (on the higher side)

 Thanks in advance

Vicky singh
Bangalore

On Tue, Dec 7, 2010 at 2:28 PM, Gabriele Sclauzero <sclauzer at sissa.it>wrote:

> 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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