[Pw_forum] Potentials between two charged particles

[Baowei Liu]

 Hi Prof. Gironcoli,

The absolute values of these negative rhos are actually very small, around
10^-7.  If I change them to be zero, I'm able to plot the charge density
with plotrho.x. I was curious why negative rho appears. Thank you for your
explanation.

Best Regards,
Baowei Liu



On Mon, Oct 4, 2010 at 2:54 AM, Stefano de Gironcoli <degironc at sissa.it>wrote:

> dear Baowei Liu ,
>  when using USPP or paw, charge density can acquire small negative
> contributions due to an incomplete  Fourier expansion of the
> augmentation charge density. This appears in particular when the system
> has large portion of vacuum (surface or molecules) since all other
> relevant contributions to the charge density vanish...
> However your cutoff values  (both  ecutwfc and ecutrho) are quite high
> so I'm a bit surprised you see negative charges.
> How large are these negative values ?
>
> stefano
>
> Baowei Liu wrote:
> > Hi Prof. Gironcoli,
> >
> > Thank you for your fast reply. I tried the following calculation. It
> gives
> > me
> >  !    total energy              =    -325.65235136 Ry.
> > But Some of rho values are negative when I do post processing. What could
> be
> > the possible reason?
> >
> >  &control
> >     calculation = 'scf'
> >     restart_mode='from_scratch',
> >     prefix='Ge',
> >     pseudo_dir = '$PSEUDO_DIR/',
> >     outdir='$TMP_DIR/'
> >  &system
> >     ibrav=  0,
> >     nat= 2, ntyp= 2,
> >     occupations = 'smearing',smearing='marzari-vanderbilt',
> >     degauss=0.05,
> >     nosym=.true.,
> >     ecutwfc = 110
> >     ecutrho = 880
> >     tot_charge = +3,
> >     assume_isolated=.true.
> >  /
> >  &electrons
> >     diagonalization = 'david',
> >     mixing_mode = 'plain',
> >     mixing_beta = 0.7
> >     conv_thr =  1.0d-7
> >     electron_maxstep=200
> >  /
> >
> > CELL_PARAMETERS cubic
> >  40.0  0.0  0.0
> >  0.0 40.0  0.0
> >  0.0  0.0 40.0
> > ATOMIC_SPECIES
> >  Ge1  72.61  Ge.pbe-paw_kj.UPF
> >  Ge2  72.61  Ge.pbe-paw_kj.UPF
> > ATOMIC_POSITIONS
> >  Ge1 0.50 0.50 0.50 0 0 0
> >  Ge2 0.5 0.50 0.70 0 0 0 0
> >
> > K_POINTS {automatic}
> >  1 1 1 0 0 0
> >
> > Thanks and Best Regards,
> > Baowei Liu
> >
> > On Sat, Oct 2, 2010 at 9:58 AM, Stefano de Gironcoli <degironc at sissa.it
> >wrote:
> >
> >
> >> the code can perform calculation for charged system using the variable
> >> tot_charge.
> >> By default the code performs the calculation for a periodic system
> >> (adding a compensating background to eliminate the G=0 divergence).
> >> In the limit of large cells this eventually converge to the isolated
> case.
> >> There are various techniques to perform calculations for isolated
> >> systems with smaller cells.
> >> Read carefully the description of assume_isolated variable in the system
> >> namelist
> >> best,
> >>
> >>   Stefano de Gironcoli - SISSA and DEMOCRITOS
> >>
> >> Baowei Liu wrote:
> >>
> >>> Dear all members,
> >>>
> >>> I am trying to calculate the potential between two ions, -- A(+2) and
> >>>
> >> A(+1)
> >>
> >>> for example. When the two ions are far away from each other, there's
> only
> >>> Coulumb potential. But when the two ions are close enough, the electron
> >>> distribution will be important for the potential.  Can QE be used to
> >>> calculate the energy or charge density for a system of two charged
> >>> particles? It will be great if QE can give the energy for different
> >>> distances. Just setting the tot_charge (=+3 in this example) is kind of
> >>>
> >> not
> >>
> >>> what I want.
> >>> Thanks in advance.
> >>>
> >>> Baowei Liu
> >>>
> >>>
> >>>
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