LAMMPS PPPM relative accuracy and timing results

Hello, I have recently ran some timing results for a ~52 K atom system using LAMMPs on stampede. I have noticed that it is very sensitive to the PPPM accuracy specified for the tests I ran. The default value is 10e-4, however I have heard mentioned before on this list (believe it was axel who said this, sorry if I am misquoting or I am taking this out of context) that this is “sloppy”, and a 10e-6 value is recommended. However, after digging through the archives I seem to be getting conflicting reports. Can someone comment on the proper method to select this parameters, or if 10e-6 is overkill? Here are the timing results.

Ran on 8 cores with a 1 MIC for 2000 steps.

61 seconds (kspace_style pppm 1.0e-4)

73 seconds (kspace_style pppm 1.0e-5)

162 seconds (kspace_style pppm 1.0e-6)

The best way to determine the PPPM accuracy needed

for your system, would be to run the same simulation

at different accuracy values and see if something important


To speed-up PPPM, you can adjust the short-range cutoff,

e.g. make it longer if PPPM is too expensive. Various other

optimizations are also possible, e.g. verlet/split, staggering, etc,

as described in Section 5.2 of the manual.