# High DFT error for HOMO-LUMO calculation

In a paper by Zhang and Musgrave it is stated that

Unfortunately, although molecular orbital (MO) theory is of immense utility, commonly used DFT functionals that can economically calculate the electronic structure of molecules may not predict orbital energies accurately.

I was wondering while DFT is an ab initio method to calculate molecular properties, why it not performing well for computing frontier molecular orbital energies.