Negative LUMO Energy During the (DFT-)computations that I have done over the last years, it happened quite often that I got negative energies for the LUMO and many times also for a bunch of virtual orbitals.

Is there a common explanation what negative energies for LUMO means?

I was told that it might have to do with the overestimation of correlation energy within DFT. But what would it mean if there is no error?
Negative compared to what?

Most chemical system have several unoccupied orbitals still in the negative energy range, meaning lower excited states are still bound systems (they don't fall just apart).