Cyclobutadiene p Molecular Orbitals
This structure has been geometry optimized before the MO's were generated. Notice that the degenerate Eg level in the D4h symmetry version are no longer degenerate. They have split into the B2g and B3g orbitals shown here. In this structure the B2g orbital is the HOMO and the B3g is the LUMO. What does this suggest about the aromaticity of cyclobutadiene?