d d t ⟨ ⟨ Q ⟩ ⟩ = − − 1 i ℏ ℏ ⟨ ⟨ ψ ψ | [ H , Q ] | ψ ψ ⟩ ⟩ = − − 1 i ℏ ℏ ⟨ ⟨ ψ ψ | ( H Q − − Q H ) | ψ ψ ⟩ ⟩ Since H | ψ ψ ⟩ ⟩ = E | ψ ψ ⟩ ⟩ then d d t ⟨ ⟨ Q ⟩ ⟩ = − − 1 i ℏ ℏ ( E ⟨ ⟨ ψ ψ | Q | ψ ψ ⟩ ⟩ − − E ⟨ ⟨ ψ ψ | Q | ψ ψ ⟩ ⟩ ) = 0 This is the reason why eigenstates of the Hamiltonian are also called stationary states.