On the position of the nitrogen 2p energy level in endohedral N@C60 (Ih)

We present results of non-empirical ROHF, UHF, and DFT calculations of endohedral N@C60 (Ih) and of its constituents, C60 (Ih) and the free N atom, in their neutral and charged forms. Based on these results, a detailed discussion is presented on the position of the nitrogen 2p energy level in N@C60. The main purpose of this work is to reveal the source of essential discrepancies in previous ROHF and DFT calculations on N@C60 (Ih). We first demonstrate the limited validity of the Koopmans' theorem exploited in previous works. It is shown that the one-electron energies in N@C60 derived at the same ROHF/6-31G* level with different quantum chemical programs (Gamess, gaussian 98, Monstergauss, Turbomole) are not constant and vary over wide limits. The value of the ionization potential from the 2p level of the encapsulated N atom was estimated in the present work by both a ΔSCF method and the Koopmans' theorem and was compared with previously reported values. We also show that ROHF and DFT methods give in fact the very similar results for N@C60 (i.e. the same ground electronic configuration and the same localization of the spin density) and discuss the known opposing conclusion on this point.