We present relativistic many-body perturbation theory plus configuration interaction calculations of the lowest four excited states of ${\mathrm{Ni}}^{12+}$, a promising candidate for highly charged ion optical clocks. By combining the convergence behavior from multiple calculation models, we perform a detailed analysis of the electron-correlation effects and both the excitation energies and their uncertainties are obtained. Our computed energies for the first two excited states deviate from experimental values by less than $10{\mathrm{cm}}^{-1}$, with relative uncertainties estimated below $0.2\%$. Building on the same computational procedure, we calculate the mass shift and field shift constants for the lowest four excited states of ${\mathrm{Ni}}^{12+}$, and the resulting isotope shifts exhibit valence-correlation-induced relative uncertainties below the $1\%$ level. These results provide essential atomic-structure input for high-precision isotope shift spectroscopy in ${\mathrm{Ni}}^{12+}$.

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• Isotope shift
• Atomic systems
• Ions
• Configuration interaction
• Hartree-Fock methods
• Perturbative methods