The potential Φ Φ 1 ( r ) of the first (central) charge distribution may be expanded in exterior multipoles Φ Φ ( r ) = 1 4 π π ε ε ∑ ∑ ℓ ℓ = 0 ∞ ∞ ∑ ∑ m = − − ℓ ℓ ℓ ℓ Q 1 ℓ ℓ m ( 1 r ℓ ℓ + 1 ) 4 π π 2 ℓ ℓ + 1 Y ℓ ℓ m ( θ θ , ϕ ϕ ) where Q 1 ℓ ℓ m represents the ℓ ℓ m exterior multipole moment of the first charge distribution. Substitution of this expansion yields the formula U = 1 4 π π ε ε ∑ ∑ ℓ ℓ = 0 ∞ ∞ ∑ ∑ m = − − ℓ ℓ ℓ ℓ Q 1 ℓ ℓ m ∫ ∫ d r ρ ρ 2 ( r ) ( 1 r ℓ ℓ + 1 ) 4 π π 2 ℓ ℓ + 1 Y ℓ ℓ m ( θ θ , ϕ ϕ )