  We report ab-initio calculations for the electronic structure of organic
charge transfer salts $\kappa$-(ET)$_2$Cu[N(CN)$_2$]Br,
$\kappa$-(ET)$_2$Cu[N(CN)$_2$]I, $\kappa^{\prime
\prime}$-(ET)$_2$Cu[N(CN)$_2$]Cl and $\kappa$-(ET)$_2$Cu$_2$(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
$\kappa$-(ET)$_2$Cu[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 $\kappa$-(ET)$_2$Cu[N(CN)$_2$]Br in eclipsed and staggered configurations
on opposite sides of the metal-insulator transition.
