  The interacting shell model, a configuration-interaction method, is a
venerable approach to low-lying nuclear structure; but it is hampered by the
exponential growth of the basis dimension as one increases the single-particle
space and/or the number of active particles. Recent,
quantum-information-inspired work has demonstrated that the proton and neutron
sectors of a nuclear wave function are weakly entangled. Furthermore the
entanglement is smaller for nuclides away from N = Z, such as heavy,
neutron-rich nuclides. Here we implement a weak entanglement approximation to
bipartite configuration-interaction wave functions, approximating low-lying
levels through coupling a relatively small number of many-proton and
many-neutron states. This truncation scheme, which we put in context to other
past approaches, reduces the basis dimension by many orders of magnitude while
preserving essential features of nuclear spectra.
