A spin-density polarization index

By analogy with the Catbo overlaplike similarity measure, a spin-density polarization index (SPI) is defined. This index gives the Euclidean distance between the alpha and beta electron densities for an open-shell molecule. An extensive study of the O-2 molecule reveals that the SPI is highly dependent on the theoretical method, but is less sensitive to the basis set provided flexible extended basis sets are used. The radial distribution of the O-2 spherically averaged electron density also reveals that near the center of the bond there is an inversion of the alpha and beta electron densities and that there is a minimum in the beta electron density near the nuclei within the internuclear region. Finally, the SPI is calculated for a variety of small molecules and the results are compared with those obtained for O-2. (C) 1998 American Institute of Physics.