[xcrysden] atomic position

[Pooja Vyas]

I opened an input file which has two atomic positions(Ca 0.0 0.0 0.0, O 0.5
0.5 0.5) in X-crysden. Then it showed me a structure with 27 atoms whose
co-ordinates (read by atoms info) are as follows:
Ca 0.5 0.5 0.0
Ca 0.0 0.0 0.0
Ca 0.5 0.0 0.5
Ca 0.0 0.5 0.5
O  0.0 0.0 0.5
O  0.5 0.5 0.5
O  0.0 0.5 0.0
O  0.5 0.0 0.0
Ca 0.5 0.5 1.0
Ca 0.0 0.0 1.0
O  0.0 0.5 1.0
O  0.5 0.0 1.0
Ca 0.0 1.0 0.0
Ca 0.5 1.0 0.5
O  0.0 1.0 0.5
O  0.5 1.0 0.0
Ca 0.0 1.0 1.0
O  0.5 1.0 1.0
Ca 1.0 0.0 0.0
Ca 1.0 0.5 0.5
O  1.0 0.0 0.5
O  1.0 0.5 0.0
Ca 1.0 0.0 1.0
O  1.0 0.5 1.0
Ca 1.0 1.0 0.0
O  1.0 1.0 0.5
Ca 1.0 1.0 1.0
I want to calculate the energy due to vacancy in quantum-espresso.
So, don't I have to include all this atomic positions and then remove one
atom, while calculating energy due to vacancy?
If I include all the positions and calculate energy, it shows me the
following error,

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
     task #        17
     from check_atoms : error #         1
     atoms #   1 and #   2 differ by lattice vector (-1, 1, 0) in crystal
axis
 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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