[Pw_forum] U calculation in LDA+U

[Peng Chen]

Thanks Burak. I appreciate your suggestions and I will try.

On Tue, Jul 10, 2012 at 11:11 AM, Burak Himmetoglu <himm0013 at umn.edu> wrote:

> Hi Peng,
>
> In my experience, the process of shifting the O atom to the origin is
> recommended since it produces more stable results numerically, especially
> in the second part of r.x where the program extrapolates to larger
> supercells using the results from the starting supercell you have used.
> However, if the crystal has already very  small number of symmetries, then
> you should be able to avoid this shifting.
>
> The first three entries in filepos are the lattice vectors in units of
> alat (like the CELL_PARAMETERS when ibrav=0). The unit cell is expanded by
> a factor of 10 in the tutorial. The length scale in filepos is not
> necessary. That file is used only to compare the distances between pairs of
> atoms and the scale does not really matter. It is generally better to scale
> up only for numerical stability reasons.
>
> Best regards,
>
> Burak
>
> On Tue, Jul 10, 2012 at 10:03 AM, Peng Chen <pchen at ion.chem.utk.edu>wrote:
>
>> When O is perturbed in the case of NiO in tutorial, it
>> is re-positioned and there are 2 different kinds of O (first with 1 atom,
>> the second with 7 atoms).  Then the kind with only one O is perturbed. In
>> the tutorial, it reads "This is done to preserve the largest possible
>> number of symmetries".  Is this process necessary? Can we just run
>> computation without re-position? In that case, there is only one kind of O
>> with 8 atoms.
>>
>> In the position file "filepos", the scale of cell length is always 10,
>> even if the scale is 7.9 in NiO and 5.42 in Fe. Shall we always use 10 in
>> position file no matter what the real scale is?
>>
>> On Tue, Jul 10, 2012 at 9:37 AM, Burak Himmetoglu <himm0013 at umn.edu>wrote:
>>
>>> Hi Peng,
>>>
>>> r.x assigns the values of the response matrices only by considering the
>>> distance between atoms and whether they have the same spin and type or not.
>>> This information has to be given by the user in the positions file
>>> "filepos" read by &input_mat. If B(1) and B(2) are different types
>>> (e.g. crystallographically) you should perturb them separately, as you
>>> outlined in the procedure, and make sure that ntyp=3 (type 1 is B(1), type
>>> 2 is B(2) and type 3 is O) in the namelist &input_mat that goes into r.x
>>>
>>> Best regards,
>>>
>>> Burak
>>>
>>>
>>> On Tue, Jul 10, 2012 at 1:45 AM, Stefano de Gironcoli <degironc at sissa.it
>>> > wrote:
>>>
>>>>  I don't know the details of r.x code but I think that, physically,
>>>> what matters is the two atoms are crystallographically equivalent or not
>>>> .therefore I think space group symmetry is the relevant one.
>>>>
>>>> stefano
>>>>
>>>>
>>>> On 07/10/2012 05:54 AM, Peng Chen wrote:
>>>>
>>>> Dear All,
>>>>
>>>> In the calculation of U, how can we determine the equivalence of the ions,
>>>> by site symmetry or space group symmetry?
>>>> E.g  ABO3, B ions has 2 different site symmetries (B(1) B(2)). Shall we run
>>>> calculation on perturbing B(1) and B(2) ions or  just a B ion?
>>>>
>>>>  In order to find U in B  ions, I tried to run calculation with perturbing
>>>> just on B, and r.x gave some errors that it needs to read more data.
>>>> So I guess it needs the perturbed data from other atoms. Shall I use the
>>>> procedures listed below?
>>>> 1. run scf calculation with Hubbard_U(i)= 1.d-20
>>>> 2. run scf with perturbing on an A ion
>>>> 3.   run scf with perturbing on a B(1) ion
>>>> 4.   run scf with perturbing on a B(2) ion
>>>> 5.   run scf with perturbing on an O ion
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
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>>
>>
>> --
>>   Best Regards.
>>         Peng
>>
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-- 
  Best Regards.
        Peng
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