The σ operator acts on the new Fermi vacuum in the H A ⊗ H B Hilbert space defined by the product of two Slater determinants Φ A Φ B . The same Hilbert space has been used by Williams et al. to develop the coupled-cluster doubles model of the dispersion interaction and has been referred to as non-symmetric Fermi vacuum. The projection superoperator can also be introduced in this space: