# Spin Hamiltonian Parameters

The spin Hamiltonian parameters of α-RuCl₃ no longer come only from ab initio calculations: Bayesian optimization fits experimental magnetization curves along the b and c axes while optimizing five parameters, the Kitaev interaction K, the off-diagonal interactions Γ and Γ′, the Heisenberg interaction J, and the c-axis g-factor g_c. The cost-minimizing set is (K, Γ, Γ′, J, g_c) = (−6.0, 7.5, −0.3, −1.75, 2.3), with exchange couplings in meV. The cost function is insensitive to the absolute value of K, so magnetization data alone do not fix the energy scale of the Kitaev coupling; it depends only weakly on Γ′ and J, while the optimization favors a large positive Γ.

The spin Hamiltonian parameters of α-RuCl₃ no longer come only from ab initio calculations: Bayesian optimization fits experimental magnetization curves along the b and c axes while optimizing five parameters, the Kitaev interaction K, the off-diagonal interactions Γ and Γ′, the Heisenberg interaction J, and the c-axis g-factor g_c. The cost-minimizing set is (K, Γ, Γ′, J, g_c) = (−6.0, 7.5, −0.3, −1.75, 2.3), with exchange couplings in meV. The cost function is insensitive to the absolute value of K, so magnetization data alone do not fix the energy scale of the Kitaev coupling; it depends only weakly on Γ′ and J, while the optimization favors a large positive Γ.