There are some links here to different DMRG implementations, including C++: itp.uni-hannover.de/~jeckelm/dmrg

@Svetlana For one-sided communication, no, you have to use windows. Wolfgangs's first two approaches could be accomplished without one-sided operations. If I understand your method correctly, couldn't you use a combination of MPI_Gather/Scatter (or the vector versions)?

@Svetlana You should look at MPI_Put and MPI_Get. These allow for a process to access another process memory space without requiring both processes participate in the communication operation. I would point you to the one-sided communication section of the MPI-3 standard for more information: mpi-forum.org/docs/mpi-3.0/mpi30-report.pdf As the MPI-3 standard is still fairly new, vendors may not have all of it implemented. You may need to use a different implementation; the latest version of MPICH has one-sided communication.