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We investigate the changes in the electronic structure of copperphthalocyanine (CuPc) crystals that is caused by intercalation with potassium.This is done by means of {\it ab initio} LSDA and LSDA+U calculations of theelectronic structure of these molecular crystals. Pristine CuPc is found to bean insulator with local magnetic moments and a Pc-derived valence band with awidth of 0.32 eV. In the intercalated compound $\rm K_2CuPc$ the additionalelectrons that are introduced by potassium are fully transferred to the $e_g$states of the Pc-ring. A molecular low spin state results, preserving, however,the local magnetic moment on the copper ions. The degeneracy of the $e_g$levels is split by a crystal field that quenches the orbital degeneracy andgives rise to a band splitting of 110 meV. Molecular electronic Coulombinteractions enhance this splitting in $\rm K_2CuPc$ to a charge gap of 1.4 eV.The bandwidth of the conduction band is 0.56 eV, which is surprisingly largefor a molecular solid. This is line with the experimentally observation thatthe system with additional potassium doping, $\rm K_{2.75}{CuPc}$, is a metalas the unusually large bandwidth combined with the substantial carrierconcentration acts against localization and polaron formation, while stronglypromoting the delocalization of the charge carriers.Comment: 5 pages, 7 figures embedde

Ab initioelectronic structure and correlations in pristine and potassium-doped molecular crystals of copper phthalocyanine