[Pw_forum] ion problem

[Cyrille Barreteau]

Thanks a lot for this detailed explanation...
Hopefully I will succeed to follow this tortuous way,
else I will wait for the next release :-)

   cyrille

Giovanni Cantele wrote:

>Cyrille Barreteau wrote:
>  
>
>>Dear all
>>
>>I have a question related to this problem.
>>I am interested in the study of charged clusters.
>>If I understand well, due to to the periodicity of the plane wave codes
>>one has to substract the correct "background" in order to keep the
>>energy from going to infinity..
>>  
>>    
>>
>This is automatically done by the code, as you may read in the INPUT_PW
>documentation file:
>nelec          REAL
>               number of electron in the unit cell
>               (may be noninteger if you wish)
>               Default: the same as ionic charge (neutral cell)
>               A compensating jellium background is inserted
>               to remove divergences if the cell is not neutral
>
>So, you set nelec in the input file, and that should be enough.
>
>  
>
>>I would like to compare the energy of clusters with different
>>charges -e -2e etc... Do you have some advices to do such calculation?
>>Is it necessary to take even larger supercells than for neutral clusters?
>>  
>>    
>>
>The problem with charged clusters is that spurious interactions are 
>introduced by the
>periodic boundary conditions, so as convergence would be achieved at 
>very large supercells.
>Neverthelessm the total energy can be corrected, to get rid of such 
>interactions, as
>described by Makov-Payne, PRB (1995). Such corrections are currently 
>implemented
>in Quantum-ESPRESSO, with the only warning that, as far as I know, 
>corrections are
>applied to the total energy but are not to the forces.
>
>If you are using espresso-3.2, you should
>- run a self-consistent calculation for you charged cluster
>- run post-processing code pp.x, for obtaining the total charge density 
>(plot_num=0) in
>a suitable format
>- run dipole.x, you may read the header of PP/dipole.f90 for very short 
>explanation
>
>You finally end up with a Makov-Payne correction that must be added to 
>the total energy
>WITH its sign.
>You may try the same charged cluster with two different supercells, and 
>compare the
>two uncorrected energies, as well as the corrected ones, finding the 
>latter much more
>converged.
>
>Next releases will probably have an easier procedure. In the CVS version 
>I understand that a new variable
>(assume_isolated) has been added to the pw.x input, that, if set to 
>.TRUE., should allow to get
>the MP correction directly within the scf loop (so post-processing is 
>not mandatory).
>
>Giovanni
>
>
>  
>


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