[Pw_forum] Hubbard U Calculation for Hematite

[Matteo Cococcioni]

Dear Sai,

your figures show that the variations are actually very small and possibly
within the numerical errors these quantities are affected by. I would
certainly try to use larger ranges for the perturbation (e.g., from -0.1 to
0.1).

best,

Matteo


On Wed, May 7, 2014 at 5:07 PM, Sai Kumar Ramadugu <sramadugu at gmail.com>wrote:

> 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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