The wavefunction for a multi-electron system cannot be written explicitly and the most common approach is the "orbital approximation". This means the wavefunction of the system 𝛹 is written as a product of one-electron wavefunctions (molecular orbitals) 𝜑. It turns out that a specific type of product is neededthe Slater determinant (eq. 2.8)to account for the indistinguishability of electrons and the Pauli principle.