Let ψ ψ = G in Green's second identity, see Green's identities. Then, ∫ ∫ V [ φ φ ( x ′ ) δ δ ( x − − x ′ ) − − G ( x , x ′ ) ∇ ∇ ′ 2 φ φ ( x ′ ) ] d 3 x ′ = ∫ ∫ S [ φ φ ( x ′ ) ∇ ∇ ′ G ( x , x ′ ) − − G ( x , x ′ ) ∇ ∇ ′ φ φ ( x ′ ) ] ⋅ ⋅ d σ σ ^ ^ ′ .