Formulation of the VB wavefunction

When two ground-state H atoms are far apart, we can be confident that electron 1 is in the Is orbited of atom A, which we denote 1), and that electron 2 is in the 

Is orbited of atom B, which we denote It is a general rule in quantum 

This expression is the VB wavefunction for the bond in molecular hydrogen. For technical reasons related to the Pauli exclusion principle (see the following Justification), this wavefunction can exist only if the two electrons it describes have opposite spins. Bonds do not form because electrons tend to pair their spins bonds are allowed to form when the electrons pair their spins. 

The spatial wavefunction in eqn 10.1 does not change sign when the labels 1 and 2 are interchanged  

According to the Pauli principle (Further information 9.3), the overall wave-function of the molecule (the wavefunction including spin) must change sign when we interchange the labels 1 and 2. Therefore, we must multiply v -b(2.1) by an antisymmetric spin function of the form shown in Further information 9.3. There is only one choice  

---