# AW: [gmx-users] How do forces from PME get calculated?

Florian Dommert dommert at icp.uni-stuttgart.de
Tue Jan 15 23:15:13 CET 2013

```Hi Thomas,

There are different possibilities to derive the forces from the PME method.
Usually with SPME, the so called analytical differentiation scheme is
applied, where the gradient of the reciprocal sum is directly calculated.
This is computationally very efficient, because it requires only one iFFT,
and conserves energy. The other method is to perform the differentiation in
reciprocal space, referred to as ik differentiation. This requires 3 iFFTs,
but conserves momentum. The Ewald sum is using the latter method.
There are several articles describing the methods and testing their
accuracy. If you do an error estimate with the GROMACS tool g_pme_error, you
will get a hint to a paper, which discusses the different methods and error
estimates and refers to further literature.

/Flo
-------
Florian Dommert
Dipl. Phys.

Institut für Computerphysik
Universität Stuttgart
Allmandring 3
D-70569 Stuttgart

Tel.: 0711-68563613
Fax: 0711-68563658

> -----Ursprüngliche Nachricht-----
> Von: gmx-users-bounces at gromacs.org [mailto:gmx-users-
> bounces at gromacs.org] Im Auftrag von Schlesier, Thomas
> Gesendet: Dienstag, 15. Januar 2013 21:56
> An: gmx-users at gromacs.org
> Betreff: [gmx-users] How do forces from PME get calculated?
>
> Maybe a somewhat dumb question:
> How do forces from PME get calculated?
> In the manual (and also some textbooks) i have only found expressions for
the
> potential for PME (or ewald-summation), but no information how the forces
get
> calculated.
> One idea i have would be
> dF_i = - [ V(r_i(t)) - V(r_i(t+dt)) ] / [ r_i(t) - r_i(t+dt) ] meaning,
calculate the
> difference in the potential energy for particle i at two integration steps
and
> divide this by the distance the particle move times (-1).
> Would this be the right approach?
> Thomas--
> gmx-users mailing list    gmx-users at gromacs.org
> http://lists.gromacs.org/mailman/listinfo/gmx-users
> * Please search the archive at
> http://www.gromacs.org/Support/Mailing_Lists/Search before posting!
> * Please don't post (un)subscribe requests to the list. Use the www
interface or
> send it to gmx-users-request at gromacs.org.
> * Can't post? Read http://www.gromacs.org/Support/Mailing_Lists

```