  A quantitative and predictive microscopic theoretical framework that can
describe reactions induced by $\alpha$ particles ($^4$He nuclei) and heavier
projectiles is currently lacking. Such a framework would contribute to reducing
uncertainty in the modeling of stellar evolution and nucleosynthesis and
provide the basis for achieving a comprehensive understanding of the phenomenon
of nuclear clustering (the organization of protons and neutrons into distinct
substructures within a nucleus). We have developed an efficient and general
configuration-interaction framework for the description of low-energy reactions
and clustering in light nuclei. The new formalism takes full advantage of
powerful second-quantization techniques, enabling the description of
$\alpha$-$\alpha$ scattering and an exploration of clustering in the exotic
$^{12}$Be nucleus. We find that the $^4$He($\alpha$, $\alpha$)$^4$He
differential cross section computed with non-locally regulated chiral
interactions is in good agreement with experimental data. Our results for
$^{12}$Be indicate the presence of strongly mixed helium-cluster states
consistent with a molecular-like picture surviving far above the $^6$He+$^6$He
threshold, and reveal the strong influence of neutron decay in both the
$^{12}$Be spectrum and in the $^6$He($^6$He,$\alpha$)$^8$He cross section. We
expect that this approach will enable the description of helium burning cross
sections and provide insight on how three-nucleon forces influence the
emergence of clustering in nuclei.
