We report ab-initio calculations for the electronic structure of organic charge transfer salts κ-(ET)2Cu[N(CN)2]Br, κ-(ET)2Cu[N(CN)2]I, κ′′-(ET)2Cu[N(CN)2]Cl and κ-(ET)2Cu2(CN)3. These materials show an ordering of the relative orientation of terminal ethylene groups in the BEDT-TTF 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 endgroup relative orientations can be used to reversibly tune κ-(ET)2Cu[N(CN)2]Br through a metal-insulator transition. Using a tight-binding analysis, we show that the molecular conformations of ethylene endgroups are intimately connected to the electronic structure and significantly influence hopping and Hubbard repulsion parameters. Our results place κ-(ET)2Cu[N(CN)2]Br in eclipsed and staggered configurations on opposite sides of the metal-insulator transition.