[gmx-developers] Fwd: [gmx-revision] check for rcoulomb=rlist with PME, Ewald and PPPM

[David van der Spoel]

Berk Hess wrote:
> Michael Shirts wrote:
> 
>>> grompp now check that rcoulomb is equal to rlist with PME, Ewald and 
>>> PPPM.
>>>   
>>
>>
>> I'm a little confused as to why this change was made.  Does this mean
>> that there will be an error printed if  rlist > rcoulomb?  In my
>> experience, it was impossible to get good energy conservation  unless
>> rlist > rcoulomb, and that a specific change was made a couple
>> versions ago to -change-  from the very grompp behavior described
>> before.  As I understand it, with rlist = rcoulomb, particles drift in
>> and out, and things that should get treated with short-range
>> interations aren't . . .
>>  
>>
> The behavior of Gromacs for the PME electrostatics has never been 
> different.
> It always used all interactions up to rlist.
> It is always better to use everything up to rlist.
> Drifting out of 1 nm gives a smaller error than out of 0.8 nm.
> 
> So if you had rcoulomb<rlist,
> you should set rcoulomb=rlist and decrease ewald_rtol
> to obtain the same beta as before.
Since many people including myself use defaults for PME settings, we 
should make the defaults good enough, so why not make ewald_rtol = 1e-6 
by default?

> 
> Berk.
> 
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-- 
David.
________________________________________________________________________
David van der Spoel, PhD, Assoc. Prof., Molecular Biophysics group,
Dept. of Cell and Molecular Biology, Uppsala University.
Husargatan 3, Box 596,  	75124 Uppsala, Sweden
phone:	46 18 471 4205		fax: 46 18 511 755
spoel at xray.bmc.uu.se	spoel at gromacs.org   http://folding.bmc.uu.se
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