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 H2 and π-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 H2
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 π-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 π 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.