Reduced Influence of Electronic Correlations in a Half-Filled System: Metallic Properties and Charge Order Transition in $\kappa$-(ET)$_2$Hg(SCN) $_2$Cl

The family of $\kappa-$(ET)$_2$Hg(SCN)$_x$Cl$_{2-x}$ can be described within a model of a metal with a half-filled conductance band. Our calculations indicate that the value of the on-site Columb repulsion U is relatively small in these system. We investigate the effects of lower U on the optical properties of $\kappa$-(ET)$_2$Hg(SCN)$_2$Cl in the metallic state and below the metal-insulator transition. Optical conductivity spectra in the metallic state above 35 K show a behavior already observed for half-filled metals on the edge of the Mott insulating state: an optical transition between Hubbard bands at frequencies equal to U is present in spectra at all temperatures, being a precursor of a Mott transition, while a coherent carriers response develops below T*=150 K. We note an influence of the lower U value both on the position of the ``U-band'' and on a relatively high T*. Untypically for a half-filled system, a metal-insulator transition at 35 K is found that is of a charge-order nature. A possibility of a charge-order ground state in a half-filed system in case of low U and relatively high nearest-neighbor Columb repulsion V is discussed.