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

Florian Dommert dommert at icp.uni-stuttgart.de
Fri Jun 19 21:02:06 CEST 2009

```* 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 ?
>>
>> 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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