Quasiparticle energies for large molecules: A tight-binding-based Green’s-function approach

We present a tight-binding approach for the calculation of quasiparticle energy levels in confined systems such as molecules. The method is based on Hedin’s GW approximation, in which the self-energy is given as the product of the Green’s function (G) and the screened Coulomb interaction (W). Key quantities in the GW formalism such as the microscopic dielectric function are expressed in a minimal basis of spherically averaged atomic orbitals. All necessary integrals are either precalculated or approximated without resorting to empirical data. The method is validated against first-principles results for benzene and anthracene, where good agreement is found for levels close to the frontier orbitals. Further, the size dependence of the quasiparticle gap is studied for conformers of the polyacenes (C_4n+2H_2n+4) up to n=30.