2ND-ORDER QUANTUM SIMILARITY MEASURES FROM INTRACULE AND EXTRACULE DENSITIES

The calculation of quantum similarity measures from second-order densi ty functions contracted to intracule and extracule densities obtained at the Hartree-Fock level is presented and applied to a series of atom s, (He, Li, Be, and Ne), isoelectronic molecules (C2H2, HCN, CNH, CO, and N-2), and model hydrogen-transfer processes (H-2/H+, H-2/H-., H-2/ H-). Second-order quantum similarity measures and indices are found to be suitable measures for quantitatively analyzing electron-pair densi ty reorganizations in atoms, molecules, and chemical processes. For th e molecular series, a comparative analysis between the topology of pai rwise similarity functions as computed from one-electron, intracule, a nd extracule densities is carried out and the assignment of each parti cular local similarity maximum to a molecular alignment discussed. In the comparative study of the three hydrogen-transfer reactions conside red, second-order quantum similarity indices are found to be more sens itive than first-order indices for analyzing the electron-density reor ganization between the reactant complex and the transition state, thus providing additional insights for a better understanding of the mecha nistic aspects of each process.