Now, the ground state energy is the lowest energy possible, i.e., E n ≥ ≥ E ground . Therefore, if the guessed wave function φ is normalized: ⟨ ϕ ϕ | H | ϕ ϕ ⟩ ≥ ≥ E ground ∑ ∑ n | c n | 2 = E ground .