[gpaw-users] Ghost atoms in fd mode?

[Ask Hjorth Larsen]

Hi

2012/7/8 Kristen Kaasbjerg <kkaa at fysik.dtu.dk>:
> Hi again
>
>>>2012/7/8 Kristen Kaasbjerg <kkaa at fysik.dtu.dk>:
>>> Hi GPAW'ers,
>>>
>>> I am interested in calculating the difference in the PAW Hamiltonian including projector
>>> terms etc between two calculations differing by one removed atom in one of them. That
>>> would be relatively straight forward if I could replace the atom I would like to remove
>>> with a ghost atoms, however, since I am using fd mode that does not seem to be possible.
>>>
>>> Is the correct and in case yes, any suggestions for how to do this in a simple way?
>>>
>>> Thanks
>>> Kristen
>>
>>Do you get an error when you try to specify a ghost atom?  It doesn't
>>seem to me that there should be any fundamental problem with this.
>
> As far as I could tell from the wiki the ghost option is only mentioned in the context of
> LCAO calcuation so I haven't tried it yet. But if you are telling me that chances are that
> it should work, I will give it a shot.
>
>>(A ghost atom does not contribute to the Hamiltonian except if you
>>represent it in an atomic basis set.  Therefore I do not quite
>>understand what the purpose of this substitution is.  Do you just want
>>the Atoms objects to have the same number of atoms?)
>
> Since I am going to substract the full PAW hamiltonians from two calculations differing
> by one removed atom, my experience so far with the projector terms which have sums
> over atoms tells me that this will be straight forward if both Atoms objects contain the
> same atoms and associated setups. So this leads me to my next questions: do ghost
> atoms have the same projectors etc as the elements of the ghost atoms otherwise would
> have had?
>
> Kristen

How to subtract two "Hamiltonians" depends completely and entirely on
how the Hamiltonian is represented.  I will not be able to understand
the operation you are performing without knowing exactly the
representation in which you work.

Effectively, a ghost atom is a set of basis functions without a setup.
 Projectors are part of a setup and are thus not found on ghost atoms.
 (In practice they have, for technical reasons, one projector which
doesn't affect the calculation because its coefficient is zero)

Regards
Ask