Ionization Potential - strange results
Posted: Wed Jul 23, 2008 2:46 am
I'm trying to calculate the ionization potential and electron affinity of bulk rutile TiO2. It's just a simple test- remove or add an electron and get the energy.
I find that the energy goes up when I add an electron, while goes down when I remove an electron. So, ionization potential (M -> M+ + e-) and electron affinity (M- -> M + e-) are both negative. This is opposite to the expected trend and would suggest that my material auto-ionizes, which I'm sure it doesn't. I've tested this with another periodic DFT code and have gotten the same result.
I've tested lone atoms in big cells and get the correct behavior, though the calculated ionization potentials tend to be smaller than experimental values by 100-300 kJ/mol.
I guess these strange results may be due to the periodic boundary conditions (possibly interacting charges), or how the background compensating charge is treated.
Any ideas on this?
Aaron
I find that the energy goes up when I add an electron, while goes down when I remove an electron. So, ionization potential (M -> M+ + e-) and electron affinity (M- -> M + e-) are both negative. This is opposite to the expected trend and would suggest that my material auto-ionizes, which I'm sure it doesn't. I've tested this with another periodic DFT code and have gotten the same result.
I've tested lone atoms in big cells and get the correct behavior, though the calculated ionization potentials tend to be smaller than experimental values by 100-300 kJ/mol.
I guess these strange results may be due to the periodic boundary conditions (possibly interacting charges), or how the background compensating charge is treated.
Any ideas on this?
Aaron