[CPMD-list] background charge

[Juerg Hutter]

Hi

> Dear Dr. Hutter,
> Thanks for your kind reply. I would like to describe the system I would like to study more detailedly. In a 10*10*10 box, there are 4 water molecules which are represented by real atoms, and another 4 water molecules which are represented by their natural charge(They are treated as background point charge for the first 4 waters). The total charge value of the point charges in the box is zero.
>
>> Hi
>>
>> you might either use an external potential defined
>> on the real space grid and given in a file or
>> alternatively you can add zero-electron pseudopotentials
>> to the system.
>
> For such a system, I'd like to ask, which method might be more suitable?
> Could you please give a more detailed example?

I would use the pseudopotentials. This way you can also better control the
spill out problem mentioned by Axel.

>
>> In both cases you have to be very careful. Due to the
>> fact that plane waves are completely unbiased, the use
>> of unscreened point charges might leed to stable
>> unphysical solutions.
> Due to the problem of stable unphysical solutions, how can I know I have
> obtained a wrong or right result. Thanks a lot.
>

You have to monitor the charge density. If you get charge density
close to the positive charges you have an undesired solution.

regards

Juerg Hutter


> Best regards,
> Weijie
>> regards
>>
>> Juerg Hutter
>>
>> ----------------------------------------------------------
>> Juerg Hutter                   Phone : ++41 44 635 4491
>> Physical Chemistry Institute   FAX   : ++41 44 635 6838
>> University of Zurich           E-mail: hutter at pci.unizh.ch
>> Winterthurerstrasse 190
>> CH-8057 Zurich, Switzerland
>> ----------------------------------------------------------
>>
>>
>> On Sat, 14 Apr 2007, Weijie HUA wrote:
>>
>>> Dear All,
>>> I'd like to ask if CPMD can do calculations on systems added with background point charges.
>>>
>>> I have checked the manual and original pipemails, but didn't find topics on background charges.
>>> However,I found many discussions on EXTERNAL POTENTIAL which makes it possible to add an external
>>> electrostatic field. I'd like to ask can this keyword be used to add background point charges?
>>> If possible, how?
>>>
>>> Many thanks.
>>>
>>> Best regards,
>>> Weijie
>>>
>>>
>>>
>>> ----
>>> Weijie HUA, Ph. D. candidate
>>> Institute of Theoretical and Computational Chemistry,
>>> Key Laboratory of Mesoscopic Chemistry,
>>> School of Chemistry and Chemical Engineering,
>>> Nanjing University,Nanjing 210093,
>>> P.R.China
>>> Email:huawj at itcc.nju.edu.cn
>>> 2007-04-14
>>>
>>>
>>>
>>
> ----
> Weijie HUA, Ph. D. candidate
> Institute of Theoretical and Computational Chemistry,
> Key Laboratory of Mesoscopic Chemistry,
> School of Chemistry and Chemical Engineering,
> Nanjing University,Nanjing 210093,
> P.R.China
> Email:huawj at itcc.nju.edu.cn
> 2007-04-15
>
>
>