 Qualitative MO theory was introduced in 1928 by Robert S. Mulliken and Friedrich Hund.A mathematical description was provided by contributions from Douglas Hartree in 1928 and Vladimir Fock in 1930.Molecular orbital diagrams are diagrams of molecular orbital (MO) energy levels, shown as short horizontal lines in the center, flanked by constituent atomic orbital (AO) energy levels for comparison, with the energy levels increasing from the bottom to the top.