Multipeak negative differential resistance (NDR) molecular devices are designed from first principles. The effect of NDR is associated with the non-linear Stark shifts and the electron localization within the conductive region and contacts. Deep I(V)-curve well is formed when the aromatic molecule, containing intramolecular hydrogen bond, is connected to each lead by the double-branch contacts. This effect occurs at the same voltage where a single-junction case exhibits only a flat step in the current characteristics. The multipeak oscillations arise from the mutual effect of the Stark shifts located at the electron-rich contacts and parts of the molecule -this opens the route for further tailoring the desired properties.

We focus on two classes of organic switches operating due to the photo- or field-induced proton transfer (PT) process. By means of first-principles simulations, we search for the atomic contacts that strengthen diversity of the two swapped current-voltage (I-V) characteristics between two tautomers. We emphasize that the low-resistive contacts do not necessarily possess good switching properties. Very often, the higher-current flow makes it more difficult to distinguish between the logic states. Instead, the more resistive contacts multiply a current gear to a larger extent. The low- and high-bias work regimes set additional conditions, which are fulfilled by different contacts: (i) in the very low-voltage regime, the direct connections to the nanotubes perform better than the popular sulfur contacts, and (ii) in the higher-voltage regime, the best are the peroxide (-O-O-) contacts. Additionally, we find that the switching-bias value is not an inherent property of the conducting molecule, but it strongly depends on the chosen contacts.