[Pw_forum] LDA, PBE and PBEsol and Cohesive energy

[Elliot Menkah]

Thank Prof. Marzari you for pointing me in the right direction. I also
looked up the lectures.

And I'm ansciously waitng for the the
https://molmod.ugent.be/deltacodesdftto get back online.


On 10/23/2014 11:40 AM, Nicola Marzari wrote:
>
> 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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>>

-- 
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