  We discuss the role of clustering on monopole, dipole, and quadrupole
excitations in nuclei in the framework of the ab initio symmetry-adapted
no-core shell model (SA-NCSM). The SA-NCSM starts from nucleon-nucleon
potentials and, by exploring symmetries known to dominate the nuclear dynamics,
can reach nuclei up through the calcium region by accommodating ultra-large
model spaces critical to descriptions of clustering and collectivity. The
results are based on calculations of electromagnetic sum rules and discretized
responses using the Lanczos algorithm, that can be used to determine response
functions, and for 4He are benchmarked against exact solutions of the
hyperspherical harmonics method. In particular, we focus on He, Be, and O
isotopes, including giant resonances and monopole sum rules.
