Theoretical study of the toroidal forms of carbon and related endohedral complexes with lithium

The atomic and electron structures of toroidal carbon molecules (C₂₄₀ and two C₁₂₀ isomers) and related endohedral complexes with lithium (Li₂@C_n and Li₄@C_n) were theoretically studied using both non-empirical (3-21G basis set) and semiempirical (MNDO) calculation schemes. For the metal-containing compounds, the behavior of lithium atoms embedded into internal cavities of the carbon framework was studied using methods of molecular dynamics. It is demonstrated that the structure of electron levels of metal-containing carbon complexes exhibits an embedded state in the forbidden band, which appears due to the presence of electrons accepted from metal atoms. The position of this embedded state and the bandgap width depend both on the initial carbon structure and on the amount of metal atoms incorporated.