Scaling of ab-initio nuclear physics calculations on multicore computer architectures

We discuss the scaling behavior of a state-of-the-art Configuration Interaction code for nuclear physics on modern multicore computer architectures. In the CI approach, the quantum many-body problem is expressed as a large sparse symmetric eigenvalue problem, of which the lowest eigenvalues and eigenvectors have to be computed. We compare the performance of the pure MPI version with the hybrid MPI/OpenMP code on Cray XT4 and XT5 platforms. For large core counts (typically 5,000 and above), the hybrid version is more efficient than pure MPI.