Two-neutrino <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msup> <mml:mn>0</mml:mn> <mml:mo>+</mml:mo> </mml:msup> <mml:mo>→</mml:mo> <mml:msup> <mml:mn>0</mml:mn> <mml:mo>+</mml:mo> </mml:msup> </mml:mrow> </mml:math> dou

This work describes the first calculation of a nuclear matrix element for the two-neutrino double-beta transition ($2\ensuremath{\nu}\phantom{\rule{0}{0ex}}\ensuremath{\beta}\phantom{\rule{0}{0ex}}\ensuremath{\beta}$ ${}^{48}$Ca${\ensuremath{\rightarrow}}^{48}$Ti) within a recently developed theory framework, no-core configuration-interaction based on density-functional theory (DFT-NCCI). The authors build on a tested approach, and explicitly include nuclear deformation. A clever choice of the single-particle wave functions reduces the computational effort. The result obtained for this transition is complementary to those obtained in other nuclear frameworks, giving confidence in modeling this very rare process with DFT-NCCI. This is not only relevant for modeling $2\ensuremath{\nu}\phantom{\rule{0}{0ex}}\ensuremath{\beta}\phantom{\rule{0}{0ex}}\ensuremath{\beta}$ decay in heavier nuclei within a unified formalism but is also highly relevant for modeling the $0\ensuremath{\nu}\phantom{\rule{0}{0ex}}\ensuremath{\beta}\phantom{\rule{0}{0ex}}\ensuremath{\beta}$ decay process for which nuclear matrix elements differ significantly between various approaches.