Estimates of effective Hubbard model parameters for C20 isomers

We report on an effective Hubbard Hamiltonian approach for the study of electronic correlations in C20 isomers—cage, bowl, and ring—with quantum Monte Carlo and exact diagonalization methods. The tight-binding hopping parameter, t, in the effective Hamiltonian is determined by a fit to density-functional theory calculations, and the on-site Coulomb interaction, U/t, is determined by calculating the isomers’ affinity energies, which are compared to experimental values. For the C20 fullerene cage we estimate tcage≃0.68–1.36 eV and (U/t)cage≃7.1–12.2. The resulting effective Hamiltonian is then used to study the shift of spectral peaks in the density of states of neutral and one-electron-doped C20 isomers. Energy gaps are also extracted for possible future comparison with experiments.