[gmx-users] PME accuracy

[Vincent Ballenegger]

Vincent Ballenegger wrote:

> * 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% !!!

I should have mentionned that the above figures were obtained with 
rcoulomb = 0.9.
(The measured force is of course the projection of the force vector 
along the diagonal of the box.)

Changing rcoulomb gives (for PME with fourier_spacing=0.1 and pme_order=4):

rcoulomb=1.74 (~ half the box size)  => force = -10.728       error: 1%
rcoulomb=1.30                        => force = -10.394       error: 4%
rcoulomb=1.00                        => force =  -9.105       error: 16%

Vincent