[Pw_forum] LDA, PBE and PBEsol and Cohesive energy

[Nicola Marzari]

Dear Elliot,

Problem 1------------
the trend LDA < PBEsol < PBE is typical, so it all looks good.
The two questions are:

1) VERIFICATION: is e.g. my PBE result, done with pseudopotentials, 
correct? I
don't know, but once this website is back online
https://molmod.ugent.be/deltacodesdft you could check your PBE
results against Wien2K all electron calculations. Also, read:
http://arxiv.org/abs/1204.2733

2) VALIDATION: is PBE or PBEsol better to describe real nickel?
I also don't know. You need to calculate properties, and see which
xc approximation matches experiments better.

Problem 2------------
the energy of a single atom in a unit cell (containing only that atom)
should be the same no matter where you put the atom (you are only
translating an infinite periodic system). So, the force should be
zero no matter where it is. If it's not zero, it should be very tiny, 
and for reasons
like not having specified the symmetry of the system, not having converged
very well to the ground state, and because GGAs have some tiny numerical 
noise.

In your calculations, though, you use vc-relax - that doesn't make sense,
since you want the unit cell to remain large (and, for the same reason,
you do not need k-point sampling - so use gamma only sampling).

Maybe worth looking at homework 1 and 2 and the first few lectures of:
ocw.mit.edu/3-320S05


				nicola




On 23/10/2014 13:17, Elliot Menkah wrote:
> Hello everyone,
> Problem 1
> ---------------------------------------
> I'm computing for lattice constant and comparing the employment of
> exchange correlation ultra-soft pp such as LDA, PBE and PBEsol. I know
> LDA would underestimate the lattice parameter, PBE would overestimate it
> but PBEsol should do better than PBE.
>
> My calculations are shows LDA= 3.419 PBE= 3.516 PBEsol = 3.455 Angstom.
> experimnetal value I'm comparing it to is 3.522 Ang
>
> PBEsol gave a lower atomic lattice value than PBE.
>
> What could be wrong with the composition of the system?
>
> Attached is the input file(vc_ni-bulk-pbesol.in) for the PBEsol USPP
> calculations.
>
> Problem 2
> -------------------------------------------
> Trying to find Cohesive energy and I already have my energy of the bulk.
> I'm now computing for the energy of a single atom in a vacuum system.
>
> I added 15 15 15 Ang to the XYZ Cell parameters and the single atom sits
> in a corner whiles I expect it to sit some where in the middle.
>
> What I'm I not doing right please.
>
> Attached is the respective (evac-ni.in) file too.
> Thank you.
>
>
>
>
> Kind Regards,
>
>
>
> Elliot
>
> --
> Elliot S. Menkah
> Research Student - Computational Chemistry/ Computational Material Science
> Theoretical and Computational Chemistry
> Dept. of Chemistry
> Kwame Nkrumah UNiversity of Sci. and Tech.
> Kumasi
> Ghana
>
> Tel: +233 243-055-717
>
> Alt Email:elliotsmenkah at gmail.com
>             elliotsmenkah at hotmail.com
>
>
>
>
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-- 

----------------------------------------------------------------------
Prof Nicola Marzari, Chair of Theory and Simulation of Materials, EPFL