For homonuclear diatomics, it is convenient(particularly for spectroscopic discussions to signify the symmetry of the molecular orbitals with respect to their behavior under inversion through the center of the molecule. The operation of inversion consists of starting at an
arbitrary point in the molecule, traveling in a straight line to the center of the molecule. and then continuing an equal distance out on the other side of the center. The orbital is designated g (for gerade , even) is identical under and u(for ungerade, odd) if it changes sign. Thus, a bonding α orbital is g and an antibonding α orbital is u (20). On the other hand, a bonding π orbital is u and an antibonding π orbital is g(21). These labels have from the αu orbitals, and similarly for the T orbitals