Mechanical Control of Molecular Conductance and Diradical Character in Bond Stretching and π-Stack Compression

This is a longish, theoretical paper dealing with the molecular conductance of H-2 and pi-stacked ethylene. At different levels of theory ranging from single-determinant to multireference, from open boundary to periodic boundary, from semiempirical to ab initio, from Green's function theory to graph theory, and from localized atomic orbitals to plane waves, the molecular junctions of H-2 and ethylene were calculated and analyzed. It was found based on simplistic models as well as sophisticated, higher-level simulations that moderately stretching the H-H bond or compressing the ethylene pi-stack increases not only the diradical character of these systems but also their conductance in the range where these two parameters show a positive correlation. Negative correlation is also observed under extreme stretching of the H-H bond or extreme compression of the ethylene pi stack. Challenges in experimental realization of the proposed molecular junctions and verification of the theoretical predictions were discussed. Digressions seen here and there in this paper may be informative and taken as a demonstration of theoreticians' way of applying insight gained from a simplistic model to a realistic system.