Inelastic effects in charge transfer and transport

In this thesis, we study several aspects of electron transfer (ET) in and charge transport (CT) through molecules. Physical intuition suggests that these two processes are similar at some level, although the theoretical methods applied are seemingly different. We establish the connection between the non-equilibrium Green's functions of CT theory and the Marcus equation for ET and show that under certain assumptions it is possible to define a stationary current that allows for the computation of ET rate. For ET, we examine the effects of anharmonicity in molecular vibrational modes on the ET rate within a Fermi's Golden Rule approach and find significant deviations from the usual harmonic picture in certain regimes. We also present a new method for the calculation of the electronic coupling matrix element based upon Boys localization and show that this converges to the generalized Mulliken-Hush method when only two charge centers exist. Next, we address the surprisingly large intersystem crossing yields in intramolecular chromophore-radical systems with a model Hamiltonian, and we explore the effect of different energetic parameters on the rate of triplet formation.;In the second half of the thesis, we turn to CT through molecular wires, and our focus is on cases where the Landauer approximation of elastic tunneling fails. We first address switching and negative differential resistance (NDR) behavior in molecular junctions; we combine electronic structure calculations with a simple polaron model to explain the molecular basis for the experimentally observed NDR and hysteresis in oligo(phenylene ethynylene) systems. We then present a new method for calculating CT in terms of many-body molecular states using non-equilibrium Hubbard Green's functions and perform conductance calculations on the classic benzene-dithiol system. Finally, we examine the potential for chiral effects in transport through helical wires using scattering theory within the first Born approximation, and we find spin-polarization can lead to slightly asymmetric transport in helical molecules. In summary, we explore a rich space of transport phenomena that lie outside the Landauer regime. A more formally challenging treatment of conductance yields qualitatively different physics.