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). 


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Professor Zipse from the LMU Munich (who does computational chemistry among other things) has no problems in presenting negative energies of virtual orbitals [for the allyl cation system](http://www.cup.lmu.de/oc/zipse/allyl-kation-c3h5.html) on his webpage. I think there is really no problem.


(The same would be true for the benzyl cation when taking the [benzyl anion](http://www.cup.lmu.de/oc/zipse/benzyl-anion-c7h7.html) as a starting assumption, since the HOMO would be emptied, however there is also a change in orbital energies involved when changing the charge do I carried on looking until I found the allyl cation, too.)


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