counter charge

Axel akoh... at
Sat Apr 18 14:48:26 UTC 2009

On Apr 17, 10:55 am, Jun <chen... at> wrote:
> hi,
> I thought I can put some classical point charges (fractional) in a
> cell using Quickstep. But I can't find the way out.

those fractional charges could be provided as pseudopotentials.
there are DFT codes that allow you to have "fractional hydrogens"
to terminate polar surfaces.

since cp2k also supports all electron calculations, you could add an
all-electron entry with a fractional charge and then define an empty
basis set for that atom type.

however, i would be very careful with a setup of that kind. if the
"fractional atom" is too close to other atoms you may get spill-out,
and many people that program quantum codes tend to implicitly or
assume that electrons and charges are integer throughout and thus
you may get unexpected results and should run careful tests on easy
to debug test systems. i am certain that cp2k developers would be
happy to get feedback and test inputs on that subject.

other than that, it might be easier to discuss options, if you could
produce an example input illustrating your setup and ideally also an
example input for a code that does support what you want to do, or
a paper describing such a calculation.


> Is that possible in QS? I guess QM/MM can do this. Since I have no
> experience on it, I would like not to go that far.

> Many thanks in advance.
> Cheers,
> Jun
> On Apr 9, 3:08 pm, Teodoro Laino <teodor... at> wrote:
> > Just a remark:
> > of course the energy of an isolated charged system is well defined.
> > It is not defined in case of a periodic charged system (without
> > considering the
> > neutralizing background charge).
> > cheers,
> > Teo
> > Teodoro Laino wrote:
> > > Dear Jun,
> > > Just to avoid confusion, the neutralizing background charge is not
> > > something  manually added  in CP2K because
> > > we don't like to have charged systems.. It comes out of the box from
> > > the EWALD summations and there's
> > > nothing you can do to avoid it (the energy of a charged system would
> > > terribly diverge!!!)...
> > > unless... unless your system is not charged..
> > > The answer to your question is : no, it is not possible. And even if
> > > it would be possible, it may end up in problematic
> > > issues.
> > > Think about putting counterions, instead, in your simulation if you
> > > don't like to have charged systems.
> > > Teo
> > > Jun wrote:
> > >> Hi all,
> > >> I want to calculate a system with a net charge. I know cp2k will
> > >> automatically apply a homogeneous neutralizing background charge with
> > >> opposite sign in the simulation cell. Is it possible in cp2k to define
> > >> an area with any shape (e.g. sheet or slab) in which the counter
> > >> charge is evenly dispersed? Or any other way to do this?
> > >> Many thanks, and happy Easter holiday.
> > >> Jun

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