[Pw_forum] Constrained occupation

[Stefano de Gironcoli]

Dear Jie,
   as Gabriele was saying constrained DFT is not implemented in PWscf: 
it is not so easy to force occupation of specific atomic-like levels in 
a plane-wave basis.
   Something related to this issue had to be considered when with Matteo 
Cococcioni  we devised a way to calculate the U parameter in DFT+U 
approaches in a consistent way in a planewave basis set [see Cococcioni 
M, de Gironcoli S, PRB 71, 035105 (2005)]. The trick has been to force 
the occupation by adding localized potentials using atomic-projectors 
multiplied by some parameter alfa_I, where I is the index of the 
perturbed atom.  In this way the energy as a function of alfa_I is the 
Legendre transform of the energy as a function of n_I (the occupation of 
the localized orbitals of atom I). Hence, by chosing an appropriate 
value of alpha_I you can control n_I. 
   Variables associated with this features in the pw.x input are 
Hubbard_U and Hubbard_alpha; you may want to have a look if these may be 
useful to your purpose.

   Hope this helps,

stefano

Gabriele Sclauzero wrote:
> Dear Jie,
>
>     The only result that you will get by insistently repeating your question every few hours is to irritate the other users, hence the probability that you get an answer will drop considerably. Also consider that from an european point of view you asked your question in the middle of the night.
>
>
> Il giorno 06/nov/2010, alle ore 03.50, Jie Jiang ha scritto:
>
>   
>> Dear All,
>>
>> Again, is it possible to do occupation constraint DFT calculations with PWscf?
>> Can the option 'occupations=from_input' do that?
>>     
>
> This option is to fix occupation, not to constrain them, and it is usually used to force an electronic configuration for an atom or a molecule computed with plane waves in a big box.
>
> Constrained DFT is another thing, in my opinion, and I don't think it has been implemented in QE so far.
>
>   
>> If not, is there any suggestion on where and how to modify to include
>> this function.
>>     
>
> You should first read the relevant literature (if I remember well, Dederichs and Bluegel were the first to apply it), then devise an algorithm and implement it. If you are really willing to do so, I think you'll get some help from the core developers. Anyway, I don't think it is so straightforward to do.
>
> Regards,
>
> GS
>
>   
>> Thanks,
>>
>> Jie   
>>
>> On Fri, Nov 5, 2010 at 7:20 PM, Jie Jiang <jiejiangnc at gmail.com> wrote:
>> Dear All,
>>
>> Does 'occupations=from_input' mean occupations of individual states
>> are fixed ?
>>
>> Can I define the occupations of the highest occupied state and lowest 
>> unoccupied state to be 1 and 1 to calculate the electron-hole excitation
>> energy?
>>
>> Thanks for any comments.
>>
>> Jie
>>
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>>     
>
>
> § Gabriele Sclauzero, EPFL SB ITP CSEA
>    PH H2 462, Station 3, CH-1015 Lausanne
>
>
>   
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