[Pw_forum] U calculation in LDA+U

[Matteo Cococcioni]

Dear Peng,

let me add one word to Burak's.
you need to distinguish two O kinds anyway, whether you shift the
atomic position or not. if you don't do that you will be perturbing
all the oxygen atoms at the same time.
The necessity to have the perturbed atom described as of a different
type (although with the same pseudopotential) is what makes it
convenient (most of the time) to place it at the origin of the unit
cell.

best,

Matteo



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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>>
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>
>
>
> --
>   Best Regards.
>         Peng
>
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-- 
Matteo Cococcioni
Department of Chemical Engineering and Materials Science,
University of Minnesota
421 Washington Av. SE
Minneapolis, MN 55455
Tel. +1 612 624 9056    Fax +1 612 626 7246