It's important to note that, when switching from { ( + , − − , − − , − − ) , l a n a = 1 , m a m ¯ ¯ a = − − 1 } to { ( − − , + , + , + ) , l a n a = − − 1 , m a m ¯ ¯ a = 1 } , definitions of the spin coefficients, Weyl-NP scalars Ψ Ψ i and Ricci-NP scalars Φ Φ i j need to change their signs; this way, the Einstein-Maxwell equations can be left unchanged.