Dear All,
I have encountered a problem in LAMMPS17Jun13 (Linux + Openmpi 1.4.5) when running minimizations on a different number of CPU processors. The problem persists also with LAMMPS10Sep13 (Mac + Openmpi 1.7.2):
For different systems/geometries I obtain minimization results which vary with the number of employed CPUs. I have attached a minimal example input and corresponding output based on the examples/dreiding.
In the examples I combined harmonic bonds and pairstyles (LennardJones also Buckingham) with longrange coulomb forces (Ewald or pppm).
When the systems are minimized, different final energies are reached when run on a different number of processors. Energy differences go up to 600 meV for the LJ based calculations.
I have deliberately suppressed reneighbouring during the minimization (via neigh_modify once yes) to exclude neighbor list effects. Also different minimizers yielded similarly different minima.
I found two possibilities, to work around this problem:

I can either shorten my pair cutoff to unphysically short distances (=2 Angstroem)

I can set the stepsize of the minimization to values around 1e4 Angstroem which will also result in same final energies (min_modify dmax 0.0001).
Could particularly the latter point to numerical instabilities in the linesearch?
Thank you very much in advance for any ideas you have on that matter.
Yours sincerely
Hendrik Heenen
Attachement.zip (104 KB)