[CPMD-list] background charge

[Weijie HUA]

Dear Dr. Kohlmeyer,
Thanks for your kind reply. We would like to study such a system. 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). I may write a pseudo input file describing the system as this:
 O                 -5.29763300   -3.25836800   -2.91750000
 O                 -3.58812700   -2.89384900   -0.22971000
 O                 -3.55474800   -0.45452000   -2.42149200
 O                 -2.35280500   -3.01605600   -4.12967500
 H                 -4.55564000   -3.52664000   -3.47156800
 H                 -6.05524300   -3.66916800   -3.33792100
 H                 -4.45248400   -2.97854900   -0.64367500
 H                 -2.99664400   -2.77746700   -0.98094100
 H                 -2.75429100   -0.73816800   -2.87745900
 H                 -4.21137000   -1.08978000   -2.72240800
 H                 -1.90252100   -2.36247300   -4.67284300
 H                 -1.63845400   -3.36637700   -3.58536900
 Tv                10.0           0.0           0.0 
 Tv                 0.0           10.0          0.0
 Tv                 0.0           0.0           10.0

-3.02412200    1.06388700    4.02619000   -0.933240
-2.82259000    1.69725800    4.72300800    0.471300
-3.38542200    1.62099000    3.33042600    0.462700
-3.23810300    4.54406300   -3.25711400   -0.908190
-2.75924300    4.40261900   -2.43598800    0.452580
-4.11634900    4.21080600   -3.05610500    0.454630
-2.67550000    2.98563100    1.31909200   -0.941290
-3.35677600    3.39450100    0.77581400    0.466950
-2.45746500    2.19264900    0.81672400    0.472400
-4.10912100    2.32214700   -1.32736200   -0.932390
-3.28100100    2.01115500   -1.71159300    0.472520
-4.66828100    1.54186100   -1.38019800    0.460030
The first 12 lines are the coordinates of the four real atoms, the following 3 lines are the translational vector. The last 12 lines are the distribution of the point charges in the format of [ x y z charge].

I really tried to use some softwares using gaussian basis sets, they can treat background charges for isolated systems, but not the periodic systems yet. So I turned to try this in CPMD. 

I would like to ask how to write a CPMD input file (e.g. the format of the potential file) for such a purpose .

And is there any criterion to judge whether the result I would obtain is right or wrong due to the 'spillout' of the electrons?
Thanks for your time and effort.

Best regards,
Weijie	

>On Sat, 14 Apr 2007, Weijie HUA wrote:
>
>WH> Dear All,
>WH> I'd like to ask if CPMD can do calculations on systems added with
>WH> background point charges.
>
>in principle, yes. but it is usually not a good idea,
>since you can run into electron 'spillout'. unlike with
>gaussian (i.e. atomic centered) basis sets, where the
>electrons are confined to the area covered by the basis
>functions, with a plane wave basis, electrons can 'go
>everywhere' in the simulation cell.


>WH> I have checked the manual and original pipemails, but didn't find
>WH> topics on background charges. However,I found many discussions on
>WH> EXTERNAL POTENTIAL which makes it possible to add an external
>WH> electrostatic field. I'd like to ask can this keyword be used to add
>WH> background point charges?  If possible, how?
>
>depending on what kind of problem you want to investigate
>(the more you tell us about what you plan to do, the better
>we can help you!), looking into one of the QM/MM interfaces
>might be an alternative (they use essentially the same mechanism
>as the EXTERNAL POTENTIAL keyword, but have additional precautions
>against spillout). you'd define your point charge field as classical
>system and then the QM part as usual...




>cheers,
>  axel.
>
>WH> 
>WH> Many thanks.
>WH> 
>WH> Best regards,
>WH> Weijie	
>WH> 
>WH> 
>WH> 
>WH> ----
>WH> Weijie HUA, Ph. D. candidate
>WH> Institute of Theoretical and Computational Chemistry,
>WH> Key Laboratory of Mesoscopic Chemistry,
>WH> School of Chemistry and Chemical Engineering,
>WH> Nanjing University,Nanjing 210093,
>WH> P.R.China
>WH> Email:huawj at itcc.nju.edu.cn
>WH> 2007-04-14
>WH> 
>WH> 
>WH> 
>
>-- 
>=======================================================================
>Axel Kohlmeyer   akohlmey at cmm.chem.upenn.edu   http://www.cmm.upenn.edu
>   Center for Molecular Modeling   --   University of Pennsylvania
>Department of Chemistry, 231 S.34th Street, Philadelphia, PA 19104-6323
>tel: 1-215-898-1582,  fax: 1-215-573-6233,  office-tel: 1-215-898-5425
>=======================================================================
>If you make something idiot-proof, the universe creates a better idiot.
>
>
----
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