G. Frenking, Perspective on "Wechselwirkung neutraler atome und homoopolare bindung nach der quantenmechanik" - Heitler W, London F (1927) Z Phys 44 : 455-472, THEOR CH AC, 103(3-4), 2000, pp. 177-179
The paper of Heitler and London was the first quantum theoretical study whi
ch explains the nature of the covalent bond. It also contains the noncrossi
ng rule.