Hi Anders,

maybe my answer above was a little bit to strict with

to little information.

Question here is: can one get physically correct results with only

insert/removals and translations? If so, using mpi or not shouldn’t

change the statistics of the results at all?

Of course you are right, there are plenty ways to speed up

Monte Carlo simulations with parallelization. What I was objecting

was the idea that this would be compatible somehow with a MD-like

simultaneous force/position update for the whole system at once.

What always can be done is distribution of the workload of one

step (distance calculation, energy calculation etc.) by means

of OpenMP.

Gedankenexperiment: you simulate a somehow tightly packed sphere

system with 1000 spheres on 1000 threads. The 50% removal-threads

would compute the acceptance probability of a removal, which should

be, depending on the potential strength (total V positive), in

the 99% range for removal-acceptance for the metropolis test in

each thread - because each thread "sees" the surroundings of his

particles as "tightly packed". After the next step, the density

of the system would be at about 50% compared to the former step,

which is possibly not a correct physical phase-space trajectory

step for this system.

But: now the real problems will start. In the next step, of the 50%

of threads that throw the "insertion" dice, any single thread will

see a lot of holes left by the erroneous removal and possibly put

new atoms there without any chance to know what the other insertion

threads are up to. The removal threads in this step will continue

to blow out atoms because any existing atom is still tightly packed.

And so on. This would be a wildly-fluctuating phase-space trajectory

without any meaningful equilibrium convergence, IMHO.

I didn’t quite understand which part you said was problematic here.

Could you elaborate?

Some years ago, J.A.Anderson, the author of HOOMD, published an approach

for an elaborate parallelization scheme with volume subdivision and

independent cell updates: doi.org/10.1016/j.jcp.2013.07.023

which also contains some thoughts and warnings for some pitfalls.

Thanks & Regards

M.