[gmx-developers] suggestion for PME for 3.3.1 for free_energy=yes

[Berk Hess]

Hi,

I did some accuracy tests on a rhombic dodecahedron (3 nm).
With rcoulomb=1, ewald_rtol=1e-6, pme_order=4.
You can see that equal grid numbers are prefered.

nx*ny*nz, nx/ny, nz, RMS error with a grid of 64^3 (in kJ/mol nm)
2560 16 10 28.4557
3072 16 12 21.8189
3584 16 14 11.9407
4096 16 16 8.80569
3888 18 12 20.9244
4536 18 14 10.1648
5184 18 16 5.86285
5832 18 18 4.54341
5600 20 14 9.78201
6400 20 16 5.21705
7200 20 18 3.47938
8000 20 20 2.70206
7744 22 16 4.9004
8712 22 18 3.15134
9680 22 20 2.2014
10648 22 22 1.74797
10368 24 18 3.01268
11520 24 20 1.97726
12672 24 22 1.40936
13824 24 24 1.12307
13520 26 20 1.83786
14872 26 22 1.19105
16224 26 24 0.852791
17576 26 26 0.749445
17248 28 22 1.11295
18816 28 24 0.729732
20384 28 26 0.595822
21952 28 28 0.50345