σx and σu are respectively the bonding and anti-bonding molecular orbitals formed by linear combination of two 1s atomic orbitals of H- atom. The spatial part of a purely covalent wavefunction for H2 molecule obtained according to molecular orbital theory is
1
σg(1)σg(2) + σg(1)σu(2)
2
σg(1)σg(2) - σg(1)σu(2)
3
σg(1)σg(2) + σu(1)σu(2)
4
σg(1)σg(2) - σu(1)σu(2)