Strength of Kitaev interaction in Na$_3$Co$_2$SbO$_6$ and Na$_3$Ni$_2$BiO$_6$</inline-formula>

The Kitaev spin liquid is proposed to be a promising realization in low spin-orbit coupling $3d$ systems, represented by Na$_3$Co$_2$SbO$_6$ and Na$_3$Ni$_2$BiO$_6$. However, the existence of Kitaev interaction is still debatable in experiments, and obtaining the strength of Kitaev interaction from first-principles calculations is also challenging. Here, we report the state-dependent anisotropy of Kitaev interaction, based on which a convenient method is developed to rapidly determine the strength of Kitaev interaction. Applying such a method and density functional theory calculations, it is found that Na$_3$Co$_2$SbO$_6$ with $3d^7$ configuration exhibits considerable ferromagnetic Kitaev interaction. Moreover, by further applying the symmetry-adapted cluster expansion method, a realistic spin model is determined for Na$_3$Ni$_2$BiO$_6$ with $3d^8$ configuration. Such a model indicates a negligibly small Kitaev interaction, but it predicts many properties, such as ground states and field effects, which are consistent with measurements. Furthermore, we demonstrate that the heavy elements, Sb or Bi, located at the hollow sites of the honeycomb lattice, do not contribute to the emergence of Kitaev interaction through proximity, contrary to common belief. The presently developed anisotropy mapping method will be beneficial not only for computations but also for measurements.