# [gmx-users] PME accuracy

Jason DeJoannis jdejoan at emory.edu
Wed May 7 15:48:01 CEST 2003

```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?

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
>
> _______________________________________________
> gmx-users mailing list
> gmx-users at gromacs.org
> http://www.gromacs.org/mailman/listinfo/gmx-users
> Please don't post (un)subscribe requests to the list. Use the
> www interface or send it to gmx-users-request at gromacs.org.
>

```