  Nuclei are prototypes of many-body open quantum systems. Complex aggregates
of protons and neutrons that interact through forces arising from quantum
chromo-dynamics, nuclei exhibit both bound and unbound states, which can be
strongly coupled. In this respect, one of the major challenges for
computational nuclear physics, is to provide a unified description of
structural and reaction properties of nuclei that is based on the fundamental
underlying physics: the constituent nucleons and the realistic interactions
among them. This requires a combination of innovative theoretical approaches
and high-performance computing. In this contribution, we present one of such
promising techniques, the ab initio no-core shell model/resonating-group
method, and discuss applications to light nuclei scattering and fusion
reactions that power stars and Earth-base fusion facilities.
