143 9 Resonance Assistance and Cooperativity in Halogen-Bonded Complexes Previously appeared as Covalency in Resonance-Assisted Halogen Bonds Demonstrated with Cooperativity in N-Halo-Guanine Quartets L. P. Wolters, N. W. G. Smits, C. Fonseca Guerra Phys. Chem. Chem. Phys. 2015, 17, 1585–1592 9.1 Introduction With the computational work presented in this chapter, we continue the analyses of the nature of halogen bonds. The detailed studies on small model complexes in the previous chapter already showed that hydrogen bonds and halogen bonds arise due to a very similar bonding mechanism, consisting of electrostatic attraction and a significant contribution from charge transfer in the σ electron system. Here, we demonstrate that this similar bond- ing mechanism has equivalent consequences for their bonding characteristics. To this end, we move to much larger model systems, namely the hydrogen-bonded Watson-Crick DNA base pairs adenine-thymine (AT) and guanine-cytosine (GC), as well as the hydro- gen-bonded guanine and xanthine quartets (G4 and Xan4, respectively). Gilli et al. proposed that hydrogen bonds between DNA bases are reinforced by resonance assistance, due to electron delocalization in the π electron system. Therefore, these hydrogen bonds are often referred to as ‘resonance-assisted hydrogen bonds’ (RAHB). [361] In previous work [114] it has been theoretically established that the hydrogen bonds in Watson-Crick DNA base pairs arise due to roughly equal contributions from electrostatic attraction and charge transfer, and that indeed the π electrons provide some additional stabilization. These findings have later been confirmed by others. [132] Furthermore, the interplay between the delocalization in the π electron system and the donor-acceptor interactions in the σ electron system was