Revisiting the Calculation of I/V Profiles in Molecular Junctions Using the Uncertainty Principle

Ortiz and Seminario (J. Chem. Phys. 2007, 127, 111106/1-3) proposed some years ago a simple and direct approach to obtain I/V profiles from the combination of ab initio equilibrium electronic structure calculations and the uncertainty principle as an alternative or complementary tool to more sophisticated nonequilibrium Green's functions methods. In this work, we revisit the fundamentals of this approach and reformulate accordingly the expression of the electric current. By analogy to the spontaneous electron decay process in electron transitions, in our revision, the current is calculated upon the relaxing process from the "polarized" state induced by the external electric field to the electronic ground state. The electric current is obtained from the total charge transferred through the molecule and the corresponding electronic energy relaxation. The electric current expression proposed is more general compared with the previous expression employed by Ortiz and Seminario, where the charge variation must be tested among different slabs of atoms at the contact. This new approach has been tested on benzene-1,4-dithiolate attached to different gold clusters that represent the contact with the electrodes. Analysis of the total electron deformation density induced by the external electric voltage and properties associated with the electron deformation orbitals supports the conclusions obtained from the I/V profiles.