[gmx-users] PME/Ewald

[mn2 at hw.ac.uk]

Hi Jason,

Thank you very much for your very informative mail. How about the accuracy 
of PME over the Ewald on a small system?   

With regards,

Abu






On Wed, 30 Mar 2005 10:50:57 -0500, Jason de Joannis <jdejoan at emory.edu> 
wrote:

> 
> > Date: Tue, 29 Mar 2005 16:19:10 +0100 (BST)
> > From: "M.Naser" <mn2 at hw.ac.uk>
> > Subject: [gmx-users] PME/Ewald
> > To: "Discussion list for GROMACS users" <gmx-users at gromacs.org>
> > Message-ID:
> <2195.137.195.36.211.1112109550.squirrel at wmail3.hw.ac.uk>
> > Content-Type: text/plain;charset=iso-8859-1
> >
> > Hi All User,
> >
> > Is there any standerd system size beyond which I should use PME
> (not
> > Ewald) to aviod overhead in setting up grids and transform?
> >
> 
>  In principle there is a break-even point because of the overhead
> involved with PME. The PME method scales as NlogN compared with
> Ewald as N^3/2 so PME prevails as you increase the number of charges
> N.
> In practice the actual value of N where performance is equal is
>  a) Small eg. <100
>  b) Hard to generalize since it depends on many things including
>     the accuracy required, the computer's architecture and the
>     FFT code.
> 
>  Moreover, the Ewald implemented in Gromacs will have a much lower
> or nonexistent break-even point because it is neither parallelized
> nor is it optimized for the selection of Ewald parameter (and
> therefore
> scales as N^2).
> 
>  Long story short, use PME even with only two charges, unless you
> don't care about performance. Probably the reason why Ewald is
> included
> is for debugging the FFT-MPI stuff.
> 
>  /Jason
> 
> -- 
> Jason de Joannis, Ph.D.
> Emory University
> 1515 Pierce Dr, Atlanta, GA 30329
> Phone: (404) 712-2983
> Email: jdejoan at emory.edu
> http://userwww.service.emory.edu/~jdejoan
> 
> 
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