Comparison of simulation methods for organic molecular systems: Porphyrin stacks

We present the results of density-functional theory based tight-binding (DF TB) and semi-empirical (PM3) Hartree-Fock (HF) calculations on the structur al and electronic properties of porphyrin molecular stacks. The geometry of the isolated molecule as well as the molecular stack has been optimized us ing the DFTB approach. The results for the single molecule are in good agre ement with nb initio calculations. The DFTB method predicts an alternating charge distribution among the carbon atoms in the porphyrin molecule, which is responsible for the formation of a tilted stacking structure. The elect ronic band structures obtained with both methods are in very good agreement . The results show a strong and non monotonic dependence of the band disper sion from the tilting angle. A transition from indirect to direct band gap is found, going from the face-to-face to the stable configuration at 25 deg rees.