 However, experimental and computational results for homonuclear diatomics from Li2 to N2 and certain heteronuclear combinations such as CO and NO show that the 3σg MO is higher in energy than (and therefore filled after) the 1πu MO.This can be rationalised as the first-approximation 3σg has a suitable symmetry to interact with the 2σg bonding MO formed from the 2s AOs.As a result, the 2σg is lowered in energy, whilst the 3σg is raised.