Molecular conductance obtained in terms of orbital densities and response functions.

Using the source-sink potential (SSP) approach recently developed in our group, we study electron transmission through molecular electronic devices (MEDs). Instead of considering the source-sink potentials exactly, we use a perturbative approach to find an expression for the transmission probability T(E) = 1 - absolute value(r(E))(2) that depends on the properties of the bare molecule. As a consequence, our approach is limited to weak molecule-contact coupling. Provided that the orbitals of the isolated molecule are not degenerate, we show that it is the orbital density, on the atoms that connect the molecule to the contacts, that largely determines the transmission through the device. Corrections to this leading-order contribution involve the second- and higher-order molecular response functions. An explicit expression for T(E) is obtained that is correct up to first order in the molecular response function. Illustrating our approach, a qualitative explanation is provided for why orders of magnitude difference in the transmission probability are obtained [M. Mayor et al., Angew. Chem. Int. Ed. 42, 5834 (2003)] upon modification of the contact position in the molecule. An extension of the formalism to interacting systems is outlined as well.