[gmx-users] The Cut-off for coulombtype heat up the water system?

[Florian Dommert]

* XAvier Periole <x.periole at rug.nl> [2009-06-20 00:45:55 +0200]:

> It is simply that if the system is not neutral the Ewald equation are  
> not valid anymore. I would guess that the original papers describe it. I 
> never looked into it.

Thanks very much. This should really be the point, because a lattice
built just of like-charged ions would have infinite energy. So this
reason should suffice to explain why Ewald can not be applied in such
cases.

Flo

>
> On Jun 19, 2009, at 21:02, Florian Dommert <dommert at icp.uni- 
> stuttgart.de> wrote:
>
>> * Mark Abraham <Mark.Abraham at anu.edu.au> [2009-06-19 14:55:02 +1000]:
>>
>>> Chih-Ying Lin wrote:
>>>> Hi
>>>> People conclude that the heating up is normal by using a plain cut- 
>>>> off.
>>>> So, how to fix the problem?
>>>
>>> 0. Do more background reading. :-)
>>>
>>>> 1. From Berk => use multiple groups.
>>>>    => how  ???
>>>>    => I have been thinking that it is better to group the molecule
>>>>    => such as: protein , non-protein
>>>>
>>>> 2. change the coulombtype without the coulomb cut-off rcoulomb ?
>>>>    => such as PME, PPPM ?
>>>>    => what's the suggestion about this?
>>>>
>>>> 3. Normally, how do people fix this problem?
>>>
>>> These days, PME will tend to be the easiest to defend in a  
>>> publication.  You will have lower heating problems with various  
>>> modified forms of  cut-offs and/or longer cut-offs, but then you  
>>> have the problem of  justifying the use of the force field, which  
>>> was probably parametrized  for some other coulomb scheme.
>>
>> Hi Mark,
>>
>> Can you please explain me or provide am reference explain why I am not
>> able to use a SPME/PME/PPPM/EwaldSum method for any kind of partial
>> charges living in an environment with periodic boundary conditions ?
>>
>> When I understood the idea of the reaction field correctly, I treat  
>> the
>> electrostatic forces with a cutoff and relative dielectric  
>> permittivity
>> != 1. With the mentionend Ewald methods I should be able to reproduce
>> exactly the same circumstances like in a reaction-field setup. So at  
>> the
>> moment I can imagine just one critical point, when using SPME/PME/PPPM
>> or an Ewald sum is the big set of parameters that have to adapted in
>> order to obtain an appropriate accuracy of the forces. In the reaction
>> field method you just have two parameters: the cutoff and epsilon_r.  
>> The
>> other algorithms require addtionally require the input of an  
>> appropriate
>> size for used grid in Fourier space and in case of SPME/PME/PPPM also 
>> an
>> interpolation order. Finally you need to set the splitting paramter
>> correctly, otherwise you will obtain unaccurate forces. So there can  
>> be
>> a very large error introduced, when applying the wrong parameters to  
>> the
>> Ewald methods. The heat up of the water is also just related to  
>> extremly inaccurate
>> electrostatic forces, since with PBC an "infinite" system is simulated 
>> and just a very small amount of the electrostatic interaction that is 
>> of
>> long range nature is calculated. Therefore an large error is not  
>> unexpected.
>> Finally the only restriction of Ewald I see is the requirement of PBC,
>> where I can reach any level of accuracy for the electrostatic force
>> given by certain charge distribution, don't I ?
>>
>> However I am always open to enhance my experience and knowledge
>> therefore please let me know if understood anything wrong.
>>
>> Flo
>>>
>>> Mark
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>>
>> -- 
>> Florian Dommert
>> Dipl.-Phys.
>>
>> Institute for Computational Physics
>> University Stuttgart
>>
>> Pfaffenwaldring 27
>> 70569 Stuttgart
>>
>> Tel: +49 - 711 / 6856-3613
>> Fax: +49 - 711 / 6856-3658
>>
>> EMail: dommert at icp.uni-stuttgart.de
>> Home: http://www.icp.uni-stuttgart.de/~icp/Florian_Dommert
>>
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-- 
Florian Dommert
Dipl.-Phys.

Institute for Computational Physics
University Stuttgart

Pfaffenwaldring 27
70569 Stuttgart

Tel: +49 - 711 / 6856-3613
Fax: +49 - 711 / 6856-3658

EMail: dommert at icp.uni-stuttgart.de
Home: http://www.icp.uni-stuttgart.de/~icp/Florian_Dommert

!! GPG-ENCODED emails preferred !!
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