where ⟨ ⟨ E ⟩ ⟩ is the average energy, ⟨ ⟨ E ⟩ ⟩ = E avg = ⟨ ⟨ ψ ψ | H | ψ ψ ⟩ ⟩ = ∑ ∑ n | c n | 2 E n . This form applies when the Hamiltonian is not time-dependent, and the ground-state energy is defined to be zero.