Firefly and PC GAMESS-related discussion club

Learn how to ask questions correctly

Davide Vanossi
vanossi.davide@unimore.it

  Davide

On Tue Jan 12 '10 11:52am, Alex Granovsky wrote
-----------------------------------------------
>Dear Davide,

>consider the following:

>1. The algorithms are somewhat different when running in parallel
>as compared with serial runs.

>2. Running in parallel using dynamic load balancing mode,
>each run is unique as the exact execution sequence will
>generally vary.

>3. Even with static load balancing, two parallel runs still can
>produce somewhat different results because MPI standard does not
>enforce any particular requirements on how collective operations
>should be implemented - and thus things like global sums etc..
>can be in theory calculated somewhat differently (i.e., using
>different order of summation) depending on the particular MPI
>implementation

>4. SCF energy is completely variational. This means that it is
>very stable with respect to things like variations in accumulation
>of round-off errors. Things like MP2 energy, any energy gradients,
>etc... etc... are much more sensitive because they are not variational.

>5. Geometry optimization is the most sensitive part as on complex
>PES, as even minor difference in computed gradients may result in
>seriously different step taken by optimizer - and hence, there are
>indeed some cases when optimization can converge even to different
>minima on PES running the same job with different number of processes.

>6. Finally, in some rare and exotic situations, there is still
>some probability that even running the same job repeatedly in
>serial, one can get slightly different answers - although this
>is usually unlikely event. One of the reasons is that Firefly
>uses run-time calibration/selection of some code (e.g., dynamic
>selection of two-electron integrals code) to achieve the best
>possible performance - and sometimes calibration can give
>different answers depending on other processes executed by OS
>as well as on the overall OS load...

>Hope this helps.

>Regards,
>Alex
>
>
>
>On Mon Jan 11 '10 3:51pm, Davide Vanossi wrote
>----------------------------------------------
>>Dear Pedro, actually I’m not worried at all :-).
>>I am just curious considering that running on a single core leads to identical output files (for different runs started with the same input file). The round-off errors seem to manifest themselves only for parallel runs when runtyp=optimize or runtyp=gradient. For other type of tasks I didn’t find any difference also running in parallel mode.
>>Anyway thank you again for your kind reply.
>>Regards.

>>   Davide Vanossi

>>On Mon Jan 11 '10 2:21pm, Pedro Silva wrote
>>-------------------------------------------
>>>An energy difference on the 9th decimal place is not relevant, neither is a problem on the 4th decimal place of the dipole moment. I guess those differences are simply caused by roundoff errors, and I would certainly not worry about them

>>>Pedro S.
>>>