  A few organic conductors show a diversity of exciting properties like Mott
insulating behaviour, spin liquid, antiferromagnetism, bad metal or
unconventional superconductivity controlled by small changes in temperature,
pressure or chemical substitution. While such a behaviour can be
technologically relevant for functional switches, a full understanding of its
microscopic origin is still lacking and poses a challenge in condensed matter
physics since these phases may be a manifestation of electronic correlation.
Here we determine from first principles the microscopic nature of the
electronic phases in the family of organic systems
kappa-(ET)2Cu[N(CN)2]BrxCl1-x by a combination of density functional theory
calculations and the dynamical mean field theory approach in a new form adapted
for organic systems. By computing spectral and optical properties we are able
to disentangle the origin of the various optical transitions in these materials
and prove that correlations are responsible for relevant features. Remarkably,
while some transitions are inherently affected by correlations, others are
completely uncorrelated. We discuss the consequences of our findings for the
phase diagram in these materials.
