  We present a detailed study of the derivation of the Hubbard model parameters
for $\kappa$-(ET)$_2$Cu$_2$(CN)$_3$ in the framework of {\it ab initio} Density
Functional Theory. We show that calculations with different (i) wavefunction
basis, (ii) exchange correlation functionals and (iii) tight-binding models
provide a reliable benchmark for the parameter values. We compare our results
with available extended H\"uckel molecular orbital calculations and discuss its
implications for the description of the properties of
$\kappa$-(ET)$_2$Cu$_2$(CN)$_3$. The electronic properties of
$\kappa$-(ET)$_2$Cu(SCN)$_2$ are also briefly discussed.
