[gmx-users] PME accuracy
vcb25 at cam.ac.uk
Tue May 6 11:35:02 CEST 2003
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?
More information about the gromacs.org_gmx-users