[Pw_forum] Potentials between two charged particles

[Stefano de Gironcoli]

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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>
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