Limit to ERI size for mpi cast

[efertitta]

It seems that above a certain size, the ERI cannot be casted from the master rank to all other. I am forced to shut down mf = mpi.scf(mf) and run the scf on each task instead. The error below implies that there is a max size of 2GB for arrays to be casted.
It appears (mpi4py/mpi4py#119) that it is a known issue that can be overcome in newer version of MPI?

File "mpi4py/MPI/msgbuffer.pxi", line 250, in mpi4py.MPI.message_simple
File "mpi4py/MPI/msgbuffer.pxi", line 511, in mpi4py.MPI._p_msg_cco.for_cco_recv
File "mpi4py/MPI/msgbuffer.pxi", line 50, in mpi4py.MPI.downcast
File "mpi4py/MPI/msgbuffer.pxi", line 495, in mpi4py.MPI._p_msg_cco.for_cco_send
OverflowError: integer 2372038530 does not fit in 'int'

[obackhouse]

by far the most annoying thing when working with mpi4py...
https://github.com/pyscf/pyscf/blob/master/pyscf/agf2/mpi_helper.py has blocking loops that make sure this limit isn't triggered - we depend on PySCF so we can use these, or just copy them into the Vayesta MPI module

[basilib]

This might be related to the issue addressed in this PR: #76. If you are broadcasting numpy arrays you can use Bcast instead of bcast to overcome this limit.

[maxnus]

mpi.scf actually already uses this wrapper - is it possible that both Bcast and bcast have the 2GB limit (to be honest it would be weird if this wasn't the case...)