The ${\mathrm{Ni}}^{12+}$ ion features an electronic transition with a natural width of only 8 mHz, allowing for a highly stable optical clock. We predict that the energy of this strongly forbidden $3s^{2}3p^4{{}^{3}P}_2\to 3s^{2}3p^4{{}^{3}P}_0$ electric quadrupole transition is $20081(10){\mathrm{cm}}^{-1}$. For this, we use both a hybrid approach combining configuration interaction with a coupled-cluster method and a pure configuration interaction calculation for the complete 16-electron system, ensuring convergence. The resulting very small theoretical uncertainty of only 0.05% allowed us to find the transition experimentally in a few hours, yielding an energy of $20078.984(10){\mathrm{cm}}^{-1}$. This level of agreement for a 16-electron system is unprecedented and qualifies our method for future calculations of many other complex atomic systems. While paving the way for a high-precision optical clock based on ${\mathrm{Ni}}^{12+}$, our theory and code development will also enable better predictions for other highly charged ions and other complex atomic systems.

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• Atomic, optical & lattice clocks
• Ions
• High-throughput calculations