[Pw_forum] ion problem
Cyrille Barreteau
cyrille.barreteau at cea.fr
Wed Nov 14 09:38:32 CET 2007
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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Cyrille Barreteau | phone : +33 (0)1 69 08 29 51
CEA Saclay | fax : +33 (0)1 69 08 84 46
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Batiment 462 |
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