Applying the extended molecule approach to correlated electron transport: important insight from model calculations

摘要

Theoretical approaches of electronic transport in correlated moleculesusually consider an extended molecule, which includes, in addition to themolecule itself, parts of electrodes. In the case where electron correlationsremain confined within the molecule, and the extended molecule is sufficientlylarge, the current can be expressed by means of Laudauer-type formulae.Electron correlations are embodied into the retarded Green function of asufficiently large but isolated extended molecule, which represents the keyquantity that can be accurately determined by means of ab initio quantumchemical calculations. To exemplify these ideas, we present and analyzenumerical results obtained within full CI calculations for an extended moleculedescribed by the interacting resonant level model. Based on them, we argue thatfor organic electrodes the transport properties can be reliably computed,because the extended molecule can be chosen sufficiently small to be tackledwithin accurate ab initio methods. For metallic electrodes, larger extendedmolecules have to be considered in general, but a (semi-)quantitativedescription of the transport should still be possible particularly in thetypical cases where electron transport proceeds by off-resonant tunneling. Ournumerical results also demonstrate that, contrary to the usual claim, the ratiobetween the characteristic Coulomb strength and the level width due tomolecule-electrode coupling is not the only quantity needed to assess whetherelectron correlation effects are strong or weak.