Geometry Optimisation with CASTEP

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The Initial Configuration of the Atoms Within the Supercell
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
%ENDBLOCK POSITIONS_ABS
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.
%BLOCK POSITIONS_FRAC
%ENDBLOCK POSITIONS_FRAC