The short answer is learn some crystallography.
Structures can have more than one unit cell representation depending on whether you use the primitive or centred cell (I centred in this case).
Thanks , julian.I got it .Learned a lot of things from you
Dear professor Julian Gale :
Sorry for bother you again.The result of elstatic constant matrix is not in correct symmetry when I convert the cubic crystal into primitive cell .
This is the result,but is not in cubic symmetry.Is there anything wrong in my input file? I generated it by materical studio
Elastic Constant Matrix: (Units=GPa)
1 212.2129 53.0239 69.4618 -23.2467 0.0000 0.0000 2 53.0239 212.2129 69.4618 23.2467 -0.0000 0.0000 3 69.4618 69.4618 195.7751 0.0000 0.0000 0.0000 4 -23.2467 23.2467 0.0000 96.0324 -0.0000 -0.0000 5 0.0000 -0.0000 0.0000 -0.0000 96.0324 -23.2467 6 0.0000 0.0000 0.0000 -0.0000 -23.2467 79.5945
This is a matter of perception - the symmetry will be correct for your elastic constants, but may not be in the form that you are expecting. You need to remember that the elastic constants are a tensor and so the values depend on your absolute choice of cell orientation. Therefore if you are comparing to literature you need to make sure that your cell has the same orientation as the paper that you are comparing to. Instead of using “cell” to specify the cell parameters, you can use “vectors” to give the Cartesian directions of the cell vectors so that they can be in exactly the orientation you want. This would be much easier if you input the centred, rather than primitive cell though & then GULP would convert it to the primitive cell with a sensible orientation.
Thanks a a lot. When I use space group and conventional cell input, gulp handle it correctly.
GULP handles it correctly in both cases - it’s just that you need to apply a tensor transformation to change the frame of reference to the one you are expecting in one case.