[gmx-developers] Re: pppm

David spoel at xray.bmc.uu.se
Thu Apr 28 08:25:09 CEST 2005

On Wed, 2005-04-27 at 16:34 -0400, Jason de Joannis wrote:
> Hi All,
>   A few comments about pppm or p3m...
> 1) It is an Ewald sum.
>      (with a k-space tweak to make it faster - like PME)
> 2) It converges.
>      (can be carried out to any accuracy, just like plain Ewald and PME)
> 3) It might be faster than PME, although they scale the same.
>      (I'm not sure there is a consensus about this in the literature)
> 4) There is a published k-space error formula.
>      (In contrast to PME. This aids parameter selection -- fewer surprises.)
Inorder to get goog accuracy you need to input a special function (Ghat)
which is generated by a separate program (I forgot which). This function
is based on box size and grid spacing. The problem is that it is not
straightforward how to adapt this function with pressure scaling. So in
practice the GROMACS implementation is not very useful. If you just
select PPPM without using the Ghat function it will work, but not be
very accurate.

> /Jason
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://xray.bmc.uu.se/~spoel

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