Having defined the supercell we can now specify the ionic positions within it. We have two choices of how to do this. We can either specify the Cartesian coordinates of each individual atom
%BLOCK POSITIONS_ABS units
Each ion occupies its own individual line. For ion number n the first item on the line is the relevant element symbol
or atomic number which is followed by a string of three numbers which are the x, y and z Cartesian
coordinates of the ion.
The second way is to use fractions of , and for the three coordinates.
After the atomic symbol or the atomic number there are three entries which represent the coordinates of the ion in terms of
fractions , and of , and respectively.