Typically, one meets such partially antisymmetric wave functions in the theory of intermolecular forces, where Ψ Ψ A ( 1 , 2 , … … , N A ) is the electronic wave function of molecule A and Ψ Ψ B ( N A + 1 , N A + 2 , … … , N A + N B ) is the wave function of molecule B. When A and B interact, the Pauli principle requires the antisymmetry of the total wave function, also under intermolecular permutations.