MPI_Cart_create creates a new communicator with the same processes as the input communicator, but with the specified topology.
The question, Who are my neighbors, can now be answered with MPI_Cart_shift:
call MPI_CART_SHIFT( comm2d, 0, 1,
nbrleft, nbrright, ierr )
call MPI_CART_SHIFT( comm2d, 1, 1,
nbrbottom, nbrtop, ierr )
The values returned are the ranks, in the communicator comm2d, of the
neighbors shifted by 
in the two dimensions.