A final thought is briefly dedicated to valence bond theory (VB), a method with less computational use, less fame, and less flashy pictures compared to MO theory, but with a longer history and a better ability for defining chemical bonds as chemists like to think about them . Indeed, if à la Lewis we consider the shared electron pair as the definition of the chemical bond, VB generates mathematical structures with a one-to-one correspondence to these pairs. If in MO theory the ground state MO of H2 is written as  = sA + sB, in VB theory the covalent bond is written as HL = sA  sB. For MO we create an orbital to put the electrons as the positive linear combination of atomic orbitals (in this case the  MO from the two s AOs). In VB we create a covalent structure for that purpose (called 'Heitler-London' in honor of the authors of the first quantum description of a chemical bond ). Note that to write the physically correct expressions we must add spins and antisymmetrize the wave functions, but for the sake of simplicity we show here only its basic term.