One may conceptualize Pauli repulsion in the following way. If the isolated-monomer wavefunctions ψ X 0 (for X = A or B) are computed by means of a variational principle, as in any SCF calculation, then ψ X 0 will adopt whatever shape minimizes its energy in isolation. Introduction of another molecule brings with it an antisymmetry constraint, so that the energy of the constrained system must be higher than that of the unconstrained system, for any finite intermolecular separation. In other words, the antisymmetry constraint forces a deformation of the wavefunction away from the minimum-energy form that it adopted in isolation, and that deformation is associated with an energy penalty (steric or Pauli repulsion). An alternative but equivalent view is that steric repulsion represents an orthogonalization penalty. Since the MOs for molecules A and B are computed in isolation, they will not be mutually orthogonal and must be deformed (orthogonalized) in order to construct a common Slater determinant that satisfies the Pauli Principle (antisymmetry requirement) for the composite AB system.