⟨ [ S → , H ^ ] ⟩ = − i ℏ ⟨ S → × ∂ H ^ ∂ S → ⟩ {\displaystyle \left\langle \left[{\vec {S}},{\hat {H}}\right]\right\rangle =-i\hbar \left\langle {\vec {S}}\times {\frac {\partial {\hat {H}}}{\partial {\vec {S}}}}\right\rangle }