Hi Prof. Gironcoli,<br><br>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.<br>
<br>Best Regards,<br>Baowei Liu<br><br><br><br><div class="gmail_quote">On Mon, Oct 4, 2010 at 2:54 AM, Stefano de Gironcoli <span dir="ltr"><<a href="mailto:degironc@sissa.it">degironc@sissa.it</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin: 0pt 0pt 0pt 0.8ex; border-left: 1px solid rgb(204, 204, 204); padding-left: 1ex;">dear Baowei Liu ,<br>
when using USPP or paw, charge density can acquire small negative<br>
contributions due to an incomplete Fourier expansion of the<br>
augmentation charge density. This appears in particular when the system<br>
has large portion of vacuum (surface or molecules) since all other<br>
relevant contributions to the charge density vanish...<br>
However your cutoff values (both ecutwfc and ecutrho) are quite high<br>
so I'm a bit surprised you see negative charges.<br>
How large are these negative values ?<br>
<font color="#888888"><br>
stefano<br>
</font><div><div></div><div class="h5"><br>
Baowei Liu wrote:<br>
> Hi Prof. Gironcoli,<br>
><br>
> Thank you for your fast reply. I tried the following calculation. It gives<br>
> me<br>
> ! total energy = -325.65235136 Ry.<br>
> But Some of rho values are negative when I do post processing. What could be<br>
> the possible reason?<br>
><br>
> &control<br>
> calculation = 'scf'<br>
> restart_mode='from_scratch',<br>
> prefix='Ge',<br>
> pseudo_dir = '$PSEUDO_DIR/',<br>
> outdir='$TMP_DIR/'<br>
> &system<br>
> ibrav= 0,<br>
> nat= 2, ntyp= 2,<br>
> occupations = 'smearing',smearing='marzari-vanderbilt',<br>
> degauss=0.05,<br>
> nosym=.true.,<br>
> ecutwfc = 110<br>
> ecutrho = 880<br>
> tot_charge = +3,<br>
> assume_isolated=.true.<br>
> /<br>
> &electrons<br>
> diagonalization = 'david',<br>
> mixing_mode = 'plain',<br>
> mixing_beta = 0.7<br>
> conv_thr = 1.0d-7<br>
> electron_maxstep=200<br>
> /<br>
><br>
> CELL_PARAMETERS cubic<br>
> 40.0 0.0 0.0<br>
> 0.0 40.0 0.0<br>
> 0.0 0.0 40.0<br>
> ATOMIC_SPECIES<br>
> Ge1 72.61 Ge.pbe-paw_kj.UPF<br>
> Ge2 72.61 Ge.pbe-paw_kj.UPF<br>
> ATOMIC_POSITIONS<br>
> Ge1 0.50 0.50 0.50 0 0 0<br>
> Ge2 0.5 0.50 0.70 0 0 0 0<br>
><br>
> K_POINTS {automatic}<br>
> 1 1 1 0 0 0<br>
><br>
> Thanks and Best Regards,<br>
> Baowei Liu<br>
><br>
> On Sat, Oct 2, 2010 at 9:58 AM, Stefano de Gironcoli <<a href="mailto:degironc@sissa.it">degironc@sissa.it</a>>wrote:<br>
><br>
><br>
>> the code can perform calculation for charged system using the variable<br>
>> tot_charge.<br>
>> By default the code performs the calculation for a periodic system<br>
>> (adding a compensating background to eliminate the G=0 divergence).<br>
>> In the limit of large cells this eventually converge to the isolated case.<br>
>> There are various techniques to perform calculations for isolated<br>
>> systems with smaller cells.<br>
>> Read carefully the description of assume_isolated variable in the system<br>
>> namelist<br>
>> best,<br>
>><br>
>> Stefano de Gironcoli - SISSA and DEMOCRITOS<br>
>><br>
>> Baowei Liu wrote:<br>
>><br>
>>> Dear all members,<br>
>>><br>
>>> I am trying to calculate the potential between two ions, -- A(+2) and<br>
>>><br>
>> A(+1)<br>
>><br>
>>> for example. When the two ions are far away from each other, there's only<br>
>>> Coulumb potential. But when the two ions are close enough, the electron<br>
>>> distribution will be important for the potential. Can QE be used to<br>
>>> calculate the energy or charge density for a system of two charged<br>
>>> particles? It will be great if QE can give the energy for different<br>
>>> distances. Just setting the tot_charge (=+3 in this example) is kind of<br>
>>><br>
>> not<br>
>><br>
>>> what I want.<br>
>>> Thanks in advance.<br>
>>><br>
>>> Baowei Liu<br>
>>><br>
>>><br>
>>> ------------------------------------------------------------------------<br>
>>><br>
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