[gmx-users] Ewald sums too slow?

[Vincent Ballenegger]

Hi all,
For small systems (<100 molecules), I would expect the standard Ewald 
sum to be faster than the PME method.
For a system of 64 SPC water molecules in a box of width L=1.2413 nm, I 
found however that gromacs-3.2.1 is about 85 times slower when using the 
standard Ewald method instead of PME !!
Why are the Ewald sums so slow?

My input parameters were:
ewald_rtol               = 1e-05
fourierspacing = 0.12
rlist = rcoulomb = rvdw = 0.62 nm
pme_order                = 6
epsilon_surface          = 0

Thanks,
Vincent