In this context, the zeroth-order wavefunction is simply a product of the wavefunctions of monomers ψ (0) = Ψ A Ψ B . However, there is a fundamental deficiency in ψ (0) such that the permutation symmetry of ψ (0) is lower than the symmetry of the total Hamiltonian, and clearly one cannot recover the exact wavefunction starting from the approximation of lower symmetry.