# 4.2. Fock space

The electronic wave function includes the spatial position for each electron () conditioned on the nuclear positions () and the spin directional coordinates for each electron (). The electronic wave function must be antisymmetric with respect to the exchange of pair particle labels because these particles are fermions. Please recall that this is the Pauli exclusion principle introduced in *Section 2.1.3*, *General formulation of the Pauli exclusion principle*.

Now, let's consider two electrons and in states and where the corresponding electronic wave function () representing the states of the two-electron system is antisymmetric () with respect to the exchange of identical pair particles :

Since a one-body electronic wave function as presented in Section 2.1, *Postulate 1 – Wave functions*, is a product of spatial () and spin components (), there are two...