We report ab initio calculations for the electronic structure of organic charge transfer salts $\kappa -{\left(\mathrm{ET}\right)}_2\mathrm{Cu}\left[\text{N}{\left(\mathrm{CN}\right)}_2\right]\text{Br}, \kappa -{\left(\mathrm{ET}\right)}_2\mathrm{Cu}\left[\text{N}{\left(\mathrm{CN}\right)}_2\right]\text{I}, {\kappa }^{″}-{\left(\mathrm{ET}\right)}_2\mathrm{Cu}\left[\text{N}{\left(\mathrm{CN}\right)}_2\right]\text{Cl}$, and $\kappa -{\left(\mathrm{ET}\right)}_2{\mathrm{Cu}}_2{\left(\mathrm{CN}\right)}_3$. These materials show an ordering of the relative orientation of terminal ethylene groups in the bis-ethylenedithio-tetrathiafulvalene molecules at finite temperature and our calculations correctly predict the experimentally observed ground state molecular conformations (eclipsed or staggered). Further, it was recently demonstrated that the ethylene end group relative orientations can be used to reversibly tune $\kappa -{\left(\mathrm{ET}\right)}_2\mathrm{Cu}\left[\text{N}{\left(\mathrm{CN}\right)}_2\right]\text{Br}$ through a metal-insulator transition. Using a tight-binding analysis, we show that the molecular conformations of ethylene end groups are intimately connected to the electronic structure and significantly influence hopping and Hubbard repulsion parameters. Our results place $\kappa -{\left(\mathrm{ET}\right)}_2\mathrm{Cu}\left[\text{N}{\left(\mathrm{CN}\right)}_2\right]\text{Br}$ in eclipsed and staggered configurations on opposite sides of the metal-insulator transition.

• Received 19 June 2015

DOI:https://doi.org/10.1103/PhysRevB.92.081109