\textit{Ab initio} electronic structure and correlations in pristine and potassium-doped molecular crystals of copper phthalocyanine
Gianluca Giovannetti, Geert Brocks, and Jeroen van den Brink

We investigate the effect that potassium intercalation has on the electronic structure of copper phthalocyanine (CuPc) molecular crystals by means of {\it ab initio} SGGA and SGGA+U density functional calculations. Pristine CuPc (in its $\alpha$- and $\beta$-structure) is found to be an insulator containing local magnetic moments due to the partially filled Cu $d$-shells of the molecules. The valence band is build out of molecular Pc-ring states with $e_g$ symmetry and has a width of 0.38/0.32 eV in the $\alpha$/$\beta$ polymorph. When intercalated to form $\rm K_2CuPc$, two electrons are added to the Pc-ring states of each molecule. A molecular low spin state results, preserving the local magnetic moment on the copper ions. The degeneracy of the molecular $e_g$ levels is lifted by a crystal field, resulting in a splitting of 52 meV between occupied and empty bands. Electronic correlation effects enhance the charge gap of $\rm K_2CuPc$ far beyond this splitting; it is 1.4 eV. The conduction band width is 0.56 eV, which is surprisingly large for a molecular solid. This finding is in line with the observed metallicity of $\rm K_{2.75}{CuPc}$, indicating that in this compound the large bandwidth combined with a substantial carrier concentration prevents polaron localization.

© 2008 The American Physical Society.