Theoretical Study of Lithium Ionic Conductors by Electronic Stress   Tensor Density and Electronic Kinetic Energy Density
  We analyze the electronic structure of lithium ionic conductors, ${\rm Li_3PO_4}$ and ${\rm Li_3PS_4}$, using the electronic stress tensor density and kinetic energy density with special focus on the ionic bonds among them. We find that, as long as we examine the pattern of the eigenvalues of the electronic stress tensor density, we cannot distinguish between the ionic bonds and bonds among metalloid atoms. We then show that they can be distinguished by looking at the morphology of the electronic interface, the zero surface of the electronic kinetic energy density. 
Hiroo Nozaki, Yosuke Fujii, Kazuhide Ichikawa, Taku Watanabe, Yuichi   Aihara, Akitomo Tachibana
physics.chem-ph cond-mat.mtrl-sci