Mulliken transfer integral

The planarity requirement for the donors and the acceptors is caused by the need for reasonable overlap between adjacent centers to facilitate larger Mulliken transfer integrals t [Eq. (8.6.12)], and therefore good electron transport. However, compact molecules with small intermolecular van der Waals distances and large overlaps may have moderate nonplanarity. 

When the fiillerene was allowed to relax, conductance is reduced drastically. Only the LUMO orbital preserves its extended character, while the rest of the orbitals become localized at the interfaces. A Mulliken population analysis and the integration of the local density of states show that the excess transferred charge ( 3.3 e) concentrates mostly at the interface hexagons. Such a finding confirms that the details of the interactions between the metal-molecule contacts are the most important factors which determine the transport properties of the molecular bridge. 

A plane wave basis set with an orbital cutoff distance of 3.9 A was used. All-electron calculations and a double numerical basis set with polarization functions (DNP) were employed. Wave function integration in reciprocal space was performed via fine grid sampling of k points with a separation of 0.02 A . For the calculation of the density of states (DOS), a 5 X 8 X 3 Monkhorst-Pack grid was used. Charge transfer was calculated based on the Mulliken Population Analysis (MPA) (Mulliken 1955). 

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