We present an ab initio calculation of the band-to-band internal-conversion rate of the $\hslash {\omega }_{\mathrm{nuc}}\approx 8.35\mathrm{eV}$ isomeric transition in ${}^{229}\mathrm{ThO}_2$. Because the nuclear transition energy exceeds the electronic band gap of ${\mathrm{ThO}}_2$, the isomer can decay nonradiatively by resonantly promoting a valence electron into the conduction band. We formulate this process as a Brillouin-zone sum over vertical interband transitions weighted by local Th-centered hyperfine matrix elements, which are evaluated directly from all-electron full-potential linearized augmented-plane-wave Bloch spinors. A finite nuclear magnetization model is included to regularize the short-range hyperfine interaction and to account for the Bohr-Weisskopf effect. After applying scissor shifts to span the experimentally reported ${\mathrm{ThO}}_2$ band gaps, we find calculated internal-conversion lifetimes in the range of $1–16\mu \mathrm{s}$. The lifetime increases strongly as band gap approaches ${\omega }_{\mathrm{nuc}}$ because the resonant interband phase space at the nuclear transition energy is reduced. For the larger reported ${\mathrm{ThO}}_2$ gaps, the calculated lifetime is comparable to the measured conversion-electron Mössbauer lifetime [Nature (London) 648, 300 (2025)]. Our analysis implies that choosing solid-state hosts with band-gap values slightly lower than ${\omega }_{\mathrm{nuc}}$ can optimize solid-state nuclear clock performance with internal conversion electron readout.

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• Fine & hyperfine structure
• Isomer decays
• A ≥ 220
• Ab initio calculations