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

$\begin{array}{cccc} \begin{displaymath} XX_{1} or I_{1} & R_{1x} & R_{1y} & ... ... & \vdots & \vdots \\ XX_{n} or I_{n} & R_{nx} & R_{ny} & R_{nz} \end{array}$

%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 $XX_{n}$ or atomic number $I_{n}$ 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 $\bm{a}$, $\bm{b}$ and $\bm{c}$ for the three coordinates.

%BLOCK POSITIONS_FRAC

$\begin{array}{cccc} \begin{displaymath} XX_{1} or I_{1} & R_{1i} & R_{1j} & ... ... & \vdots & \vdots \\ XX_{n} or I_{n} & R_{ni} & R_{nj} & R_{nk} \end{array}$

%ENDBLOCK POSITIONS_FRAC

After the atomic symbol or the atomic number there are three entries which represent the coordinates of the ion in terms of fractions $R_{1i}$, $R_{1j}$ and $R_{1k}$ of $\bm{a}$, $\bm{b}$ and $\bm{c}$ respectively.