Cluster models for lithium intercalated graphite: Electronic structures and energetics

Model compounds designed to simulate graphite and lithium intercalated graphite (LIG) have been investigated in ab initio calculations at the Hartree-Fock and Second-Order Moller-Plesset levels of theory. Several cluster models were used in attempts to calculate the interlayer distance, charge transfer and delocalization in bulk LIG. Optimized interlayer distances range from 3.56 Angstrom to 4.00 Angstrom. Each model studied supports one or several electronic states of charge transfer nature in which the Li atom carries a positive charge, and the transferred electron is more or less delocalized over the carbon pi orbitals in the graphitic portion of the cluster. There is a clear trend of HOMO-LUMO gap reduction as the size of the model cluster increases. The results for the larger of these polyaromatic hydrocarbon clusters demonstrate that this method is a viable option for studying LIG at the atomic level.