Local coordination polyhedra octahedron

The VD polyhedra model also allows one to introduce two important parameters describing distortion of the atomic coordination [16, 17], The first of these is the Da vector that originates in the A atom and ends in the centroid of the VD polyhedron. According to [18], length of the Da vector is proportional to the gradient of the local electric field created by the atoms surrounding the A atom in the structure. The second parameter is the second moment of inertia, Gs, which describes the deviation of the VD polyhedron from an ideal sphere and characterizes uniformity of distribution of the X and Z atoms around A. For an ideal sphere, G3 = 0.077, whereas for an ideal AXg octahedron (which correspond to a cubic VD polyhedron), G3 = 0.0833 [17]. 

---