This relation is explicitly given in terms of the Weyl tensor C, namely Ψ0 = C αβγδK αQ βKγQ δ = 2(Eαβ − 𝜀γδ(αsγBδβ))Q αQ β, where Eαβ = CαγβδTγT δ and Bαβ = ∗CαγβδTγTδ are the electric and magnetic parts of C with respect to the timelike vector field T = ∂t.