Visualizing covalency and multiple bonds in terms of the electronic single-particle density matrix

Two-dimensional cuts through spin-integrated single-particle density matric es (SPDMs) have proven to be a valuable tool to discuss the nature of chemi cal bonding independent of the degree of rotation of the underlying orbital s in Hilbert space. Starting from a straightforward concept for the interpr etation of sigma bonds, we briefly present how the transition from purely i onic to mainly covalent bonding is mirrored in the off-diagonal features of the SPDM in the isoelectronic series LiF, BeO and BN. Furthermore, we outl ine one of several possible ways to extend the graphical representation and interpretation of SPDMs to the case of pi bonds in organic molecules and a pply it to the conjugated pi -electron system 1,6-hexadiynediol.