PC GAMESS/Firefly-related discussion club

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sanya
sanya@photonics.ru

I'm afraid you'll have to perform a conventional DFT calculation: Ip=E(cation)-E(neutral), where E are the energies after full DFT calculations. This is because Kohn--Sham orbitals are not better than Hartree--Fock ones, they are just different orbitals, and Koopmans theorem is not valid for them. Anyway, full DFT calculation for a cation is just a little more difficult than a calculation for a neutral molecule, especially when good starting orbitals are available. However, it is still a question, whether KS orbitals of a neutral molecule are a good initial guess for the cation.