[gmx-users] PME accuracy

[David van der Spoel]

On Wed, 2003-05-07 at 15:47, Jason DeJoannis wrote:
> Hi gmx-users,
> 
>   Unfortunately, as I think it was David who pointed it
> out to me: there is no analytical error estimate for
> smooth PME yet. But as the error-estimates for Ewald, P3M
> and regular PME would suggest, the error varies with several
> things including the number of charges. Keep this in mind 
> as well as the fact that the Ewald parameter is probably 
> being adjusted "behind the scenes" in order to deliver 
> the specified "ewald_rtol".
> 
>   If anyone knows of a way to find out what Ewald parameter
> (alpha) is being used, let me know. Also I am not very clear
> on what is being done, if anything, to the PME parameters 
> behind the scenes. For example I have on occasion reduced the
> Fourier spacing to find no change was actually made in the 
> mdout.mdp file. Is that possible? or am I mistaken?
If you also specify the grid points (nkx etc) that has prescendence over
the fourier spacing.


> 
> best regards,
> 
>   Jason
> 
> ---
> Jason de Joannis, Ph.D.
> Chemistry Department, Emory University
> 1515 Pierce Dr. NE, Atlanta, GA 30322
> Phone: (404) 712-2983
> Email: jdejoan at emory.edu
> http://userwww.service.emory.edu/~jdejoan
> 
> 
> 
> Quoting Vincent Ballenegger <vcb25 at cam.ac.uk>:
> 
> > Dear gmx-users,
> > 
> > According to the gromacs manual, the PME method is such that "with a 
> > grid spacing of 0.1nm and cubic interpolation (i.e. pme_order=4), the 
> > electrostatic forces have an accuracy of 2-3e-4".
> > I did a small test and obtained the following results for the electric 
> > force between two ions (charge +e and -e) a distance 2.35nm apart along 
> > the diagonal of a cubic box of 3.5nm:
> > 
> > * Ewald method with fourier_nx=25:
> >                force = -10.824      "exact result"
> > * PME method with fourier_spacing=0.1, pme_order=4:
> >                force = -7.789         error: 28% !!!
> > * PME method with fourier_spacing=0.05, pme_order=4:  (very slow!)
> >                force = -10.932         error: 1%
> > * PME method with fourier_spacing=0.1, pme_order=6:
> >                force = -10.716       error: 1%
> > 
> > Where is the claimed accuracy of 3e-4?
> > 
> > Thanks,
> >   Vincent
> > 
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> 
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-- 
Groeten, David.
________________________________________________________________________
Dr. David van der Spoel, 	Dept. of Cell & Mol. Biology
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://xray.bmc.uu.se/~spoel
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