The archetypal example of QI is Young's double-slit a) Electronic mail: xhzheng@theory.issp.ac.cn b) Electronic mail: gsolomon@chem.ku.dk interference experiment, where light passes through slits in two barriers. The first barrier has a single slit and the second has two slits. On the wall behind the second barrier, an interference pattern is observed. This experiment is illustrated in Figure (a). Wave-particle duality means that the light displays characteristics of both classically defined waves and particles. In the double-slit experiment, the interference pattern is due to the phase difference of the waves caused by the differing lengths of the optical paths as light passes through the two slits in the second barrier. Similarly, wave-particle duality also governs the behavior of electrons. When the size of the device is comparable to the electronic phase coherence length, QI effects can manifest. This QI can change the electronic transport through molecular sized devices dramatically, as illustrated in Figure . The QI can be destructive or constructive depending on whether the phase difference between interfering paths are π or 0. We use the language of "paths" to draw an analogue with the two-slit experiment, although no such paths in physical space are evident through the molecule. While there are mathematical arguments to support the idea that the cyclic structure of a benzene ring can represent the two paths , destructive QI is also present in acyclic molecules, challenging this interpretation. Instead, QI in molecules can be understood in terms of interfering contributions between pairs of molecular orbitals 22 (MOs). While the MOs of the isolated molecule are perturbed by the coupling to the electrodes, the coupling to the electrodes is typically small. This means that the molecular conductance orbitals (MCOs) which are the eigenbasis of the molecular Hamiltonian including the self-energies of electrodes are similar to the MOs or eigenbasis of the isolated molecules. Therefore, a distinction is generally not made between MOs and MCOs in practice. When describing the MOs of organic molecules, it is useful to distinguish between the σ-and π-orbitals. Destructive QI in the π-system of conjugated molecules is frequently observed in calculations as a sharp transmission dip and has been reported in many systems, such as graphene-like molecules , π-stacked molecules and cross-conjugated molecules . The molecular orbital analysis by Gunasekaran, et al. provides a method to visualize the constructive and destructive QI between pairs of π-MOs. Typically the σ-orbitals are not considered in the methods for predicting QI in conjugated molecules as σ-orbital contribution drops off extremely rapidly with length and is only signficant in certain limited situations. Recently, destructive QI has also been identified in the σ-systems of molecules, such as saturated silanes , the Bicyclo[2.2.2]octane class of molecules and cyclo-alkanes . The QI in σ-systems has been utilized to engineer short molecules with comprehensive suppression of the electronic transmission. Given the differences in the orbital character of the π-orbitals of conjugated molecules and the σ-orbitals of saturated systems, a natural question to ask is: is the orbital picture of destructive QI in σ-systems similar to that seen in π-systems or do we expect differences?