Using Plancherel theorem, the energy can also be summed in Fourier space E ℓ ℓ r = ∫ ∫ d k ( 2 π π ) 3 ρ ρ ~ ~ TOT ∗ ∗ ( k ) V ~ ~ ( k ) = ∫ ∫ d k ( 2 π π ) 3 L ~ ~ ∗ ∗ ( k ) | ρ ρ ~ ~ u c ( k ) | 2 Φ Φ ~ ~ ( k ) = 1 Ω Ω ∑ ∑ m 1 , m 2 , m 3 | ρ ρ ~ ~ u c ( k ) | 2 Φ Φ ~ ~ ( k )