[Pw_forum] Hubbard U Calculation for Hematite

[Sai Kumar Ramadugu]

Dear Pietro:

My calculations use a unit cell of Fe2O3. Currently I am trying to optimize
2x1 and 2x2 supercells. Once I have the optimized structure, I want redo
the linear response calculations.

Dear Matteo:

The deviations are smaller. For the case of intersection at 0.0, I have
very small range from 0.00001 to 0.00025 for the difference between the
occupations at alpha=0.0 for bare and scf converged responses.

About linearity, I have attached two figures for the response of atom 5
(Fe) with respect to the perturbed atom (Fe) at the origin and atom 6 (Fe)
with respect to the perturbed atom (Fe) at the origin.
The alpha values for these two graphs are -0.01, 0.0 and +0.01.
The response of atom 5 with perturbed atom does intersect (not at 0.0) but
for the response of atom 6, the two lines do not intersect.

Would you suggest to use larger values of alpha (close to 0.1) or use a
super-cell for the calculations?

Thanks very much for the time and help.
Sai


On Wed, May 7, 2014 at 7:59 AM, Matteo Cococcioni <matteo at umn.edu> wrote:

> Dear Sai Kumar Ramadug,
>
> just another idea: how wide are the deviations from linearity? on-site
> occupations are difficult to converge beyond the 3rd digit. Sometimes, when
> they are minimally affected by the perturbation, their variation can be
> within numerical noise.
>
> regards,
>
> Matteo
>
>
>
>
> On Wed, May 7, 2014 at 2:44 PM, Pietro Bonfa' <pietro.bonfa at fis.unipr.it>wrote:
>
>> Dear Sai Kumar Ramadug,
>>
>> I can comment only on the sentence below:
>>
>> On 05/06/2014 05:47 PM, Sai Kumar Ramadugu wrote:
>> > The problem I am having as of now is that some of the responses dn0/da
>> > and dn/da are not linear and do not intersect at 0.
>>
>> It happened to me as well. In my case I think that it was due to the
>> size of my supercell (i.e. to the spurious perturbation introduced by
>> the PBC). With a bigger supercell I obtained reasonable results.
>>
>> Kind regards,
>> Pietro Bonfa'
>>
>> --
>> Pietro Bonfa' - PhD student
>> Dipartimento di Fisica e Scienze della Terra "Macedonio Melloni"
>> Viale delle Scienze 7A
>> 43124 Parma - Italy
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>
>
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