... concentration with respect to the anion A, calculated from the position of the elements in the Periodic Table; (b) AA(n/m') or C'C (this depends on the VECA value) which correspond to the average number of electrons 0108-7681/90/010007-17503.00 per tetrahedron available for bonds between anions or between central atoms (and/or for electron lone pairs on the central atom), respectively; and finally (c) C'AC ' which expresses the average ... No base tetrahedron with this code is possible. ...