Nonequilibrium Green's functions (NEGF) is the standard method for charge transport simulations in molecular electronics community. NEGF utilizes language of elementary excitations (quasi-partciles, orbitals), thus NEGF-DFT is the natural combination in analysis of junctions responses. The combination is mostly convenient in situations where quasi-partciles are non- or weakly interacting. For example, in description of elastic tunneling or off-resonant inelastic electron tunneling spectroscopy (IETS). When interactins in the system are strong (compared with molecule-contacts coupling strentgh), language of many-body states of an isolated molecule is preferabvle basis for analysis. Such situation appears in modeling of resonant IETS, Coulomb blockade regime, optical responses of molecular junctions, etc. We introduce extension of the equilibrium Hubbard Green's function technique to nonequilibrium realm of molecular junctions and discuss its application in first principles simulations. Contrary to standard NEGF, where diagrammatic explansion is performed in weak intra-system interactions, small parameter of the Hubbard NEGF diagrammatic technique is molecule-contacts couplings. Its formulation in the basis of many body states allows to avoid artificial concepts of orbitals and their rerganization on charging/excitation in the molecule and makes analysis of response in optoelectronic molecular devices at the same time keeping information on broadening and bath-induced coherences. We also show how the Hubbard NEGF allows to simulate electronic friction beyond the celebrated Head-Gordon and Tully expression and its usefulness in derivation of generalized rates. Finally, we discuss a new technique, which does not require small parameter thus going beyond diagrammatic expansions in both NEGF and Hubbard NEGF.

Nonequilibrium Hubbard Green's functions (Hubbard NEGF) for first principles simulations in molecular junctions

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

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