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-boun...@gromacs.org [mailto:gmx-users- > boun...@gromacs.org] Im Auftrag von Schlesier, Thomas > Gesendet: Dienstag, 15. Januar 2013 21:56 > An: gmx-users@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@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-requ...@gromacs.org. > * Can't post? Read http://www.gromacs.org/Support/Mailing_Lists -- gmx-users mailing list gmx-users@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-requ...@gromacs.org. * Can't post? Read http://www.gromacs.org/Support/Mailing_Lists