Asbtract: Understanding how nuclei respond to external probes is essential for connecting nuclear structure to fundamental interactions. In this talk, I will present a new framework to calculate integrated properties of the nuclear response starting directly from nuclear forces. Using the in-medium similarity renormalization group (IMSRG), we evaluate expectation values of operators that encode the multipole response of nuclei. I will show applications to the monopole, quadrupole, and dipole response in closed-shell nuclei from helium to nickel, and discuss the impact of many-body correlations beyond the random-phase approximation. Our IMSRG calculations provide an improved description of experimental data in oxygen and calcium isotopes, including a successful reproduction of the Thomas–Reiche–Kuhn sum-rule enhancement. Finally, I will outline how this approach can serve as a benchmark for other ab initio methods that describe nuclear response functions through explicit excited states.

[1] Porro, A., Schwenk, A., & Tichai, A. (2025). Impact of ground-state correlations on the multipole response of nuclei: Ab initio calculations of moment operators. arXiv preprint arXiv:2507.20665.