[gmx-users] PME accuracy

[Vincent Ballenegger]

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