[Pw_forum] Nondeterministic rounding errors?

[Norbert Nemec]

Has anyone seen something like this before:

I start two MD runs that are identical except for the initial velocities. Both are started on the same Cray machine on different but identical sets of cores, using the same binary. In my understanding, the first SCF loop should produce binary identical results. Yet - the wave function produced by pw2casino shows differences in the last bit of many floating point values.

Is there any point where non-determinism might affect rounding? Perhaps somewhere in the MPI communications?
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