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

TL;DR

This paper explores the extended molecule approach for correlated electron transport, demonstrating that accurate ab initio calculations can reliably predict transport properties, especially for organic electrodes, by analyzing model calculations.

Contribution

It introduces a model-based analysis of the extended molecule approach, showing its effectiveness for organic electrodes and providing insights into electron correlation effects in transport.

Findings

Transport properties can be reliably computed for organic electrodes using small extended molecules.

Larger extended molecules are needed for metallic electrodes, but off-resonant tunneling can still be described semi-quantitatively.

The ratio of Coulomb strength to level width is not the sole indicator of correlation strength.

Abstract

Theoretical approaches of electronic transport in correlated molecules usually consider an extended molecule, which includes, in addition to the molecule itself, parts of electrodes. In the case where electron correlations remain confined within the molecule, and the extended molecule is sufficiently large, the current can be expressed by means of Laudauer-type formulae. Electron correlations are embodied into the retarded Green function of a sufficiently large but isolated extended molecule, which represents the key quantity that can be accurately determined by means of ab initio quantum chemical calculations. To exemplify these ideas, we present and analyze numerical results obtained within full CI calculations for an extended molecule described by the interacting resonant level model. Based on them, we argue that for organic electrodes the transport properties can be reliably computed, because the extended molecule can be chosen sufficiently small to be tackled within accurate ab initio methods. For metallic electrodes, larger extended molecules have to be considered in general, but a (semi-)quantitative description of the transport should still be possible particularly in the typical cases where electron transport proceeds by off-resonant tunneling. Our numerical results also demonstrate that, contrary to the usual claim, the ratio between the characteristic Coulomb strength and the level width due to molecule-electrode coupling is not the only quantity needed to assess whether electron correlation effects are strong or weak.