Downfolding electron-phonon Hamiltonians from ab initio calculations: Application to K-3 picene

We propose an electron-phonon parametrization which is constructed to reproduce target geometry and harmonic frequencies taken from first principles calculations or experiment. With respect to standard electron-phonon models, it adds a \"double-counting\" correction, which takes into account the lattice deformation as the system is dressed by low-energy electron-phonon processes. We show the importance of this correction by studying potassium-doped picene (K-3 picene), recently claimed to be a superconductor with a T-c of up to 18 K. The Hamiltonian parameters are derived from ab initio density functional theory, and the lattice model is solved by dynamical mean-field theory. Our calculations include the effects of electron-electron interactions and local electron-phonon couplings. Even with the inclusion of a strongly coupled molecular phonon, the Hubbard repulsion prevails and the system is an insulator with a small Mott gap of approximate to 0.2 eV.