[CPMD-list] background charge

[Weijie HUA]

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?

>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.

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