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The %maxcore parameter in ORCA gives the memory per core needed for a calculation, however it is unclear to me if this should be the entire memory available to the job or if it should be some percentage of the max memory.

How would I define %maxcore if I have a node with 64GB of memory, slurm allocates 32GB of the memory to me, and I have 4 cores. Presumably it would be set to 8000 to assign 8GB to each core, but I have seen calculations fail with out of memory errors when doing this that I would not predict would have this problem.

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The ORCA Input Library website has a list of commonly encountered errors that discusses using the maxcore option.

As you described, this option does set the "max" memory per core. I put max in quotations because it seems this is the upper limit under ideal conditions and ORCA can (and apparently often does) overshoot this limit. They recommend requesting no more than 75% of the physical memory you have available, so in your case that would be 6GB per core.

I have not extensively used ORCA, so I don't know if this is a conservative recommendation or if you really will often encounter issues if you request, say, 90% of physical memory.

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