Origin of configuration instabilities in the DFT-NCCI calculation

Investigate the origin of the instabilities produced by the excluded configurations C_5^{\rm B}, C_{11}^{\rm B}, and C_{14}^{\rm B} in the DFT-NCCI calculation of the lowest I=0, T=1 state in ^{10}B and the associated isospin-symmetry-breaking correction to the superallowed ^{10}C\rightarrow^{10}B beta decay.

Background

The DFT-NCCI calculations for ^{10}B exhibit instabilities associated with three configurations, C_5^{\rm B}, C_{11}^{\rm B}, and C_{14}^{\rm B}. The most pronounced instability occurs for C_5^{\rm B}, which produces an anomalously low excitation energy for the projected T=0 state and substantially affects the calculated isospin-symmetry-breaking correction, despite having only a minor effect on the energy of the lowest I=0, T=1 state.

Because these configurations were excluded from the final ^{10}B configuration space, the resulting configuration space is asymmetric and may introduce an unphysical isospin-breaking contribution. The authors explicitly identify the physical or computational origin of these instabilities as unresolved, making their clarification necessary for a more reliable assessment of the configuration-interaction contribution to the decay correction.