Sent to CCL by: Jozsef Csontos [jozsefcsontos]|[creighton.edu]
Hi,
my short question is:
is it possible to get different optimized final structures using the
same input and architecture, OS, software (and so on), but different
number of processors?
My longer comments are:
I understand that we use approximations (in QM and in life too:) and
there might be error accumulations and our number representation
depends
on the hardware etc. But, I'm quite sure the answer is no. Maybe I'm
wrong, so I tell you the story.
The story:
I have been using a commercial quantum package, which uses the MPI
libraries (MPICH) for parallelization. I was about to map the PES of
several tripeptides in water using DFT. (Of course, the molecules are
flexible and the PESs are flat and more more approximations) I'm not
totally surprised that the results are different on 1 node with 1
processor and 6 node with 6 processors. (In the first case there is no
MPI and in the second one there is.) Interestingly, the results are
different on 3 nodes with 6 processors (2CPUs/node) and on 6 nodes with
6 processors (1CPU/node), in addition the calculation using the
previous
6 processor configuration (2CPUs/node) differs from the 4 CPUs config
(2CPUs/node) ones.
The energy differences between the structures are small about
1kcal/mol,
however, the rmsd is quite large 1.9 Angstrom considering heavy atoms
and one of these structures is the deepest minimum on the PES, so these
structures belong to the low energy region. I haven't checked all the
structures whether they are minima or other stationary points. I guess
this is not related to the question, but if all of them were minima my
question would be louder.
I'm wondering what your opinions are?
Best wishes,
Jozsef
--
Jozsef Csontos, Ph.D.
Department of Biomedical Sciences
Creighton University,
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