[Wien] why artificial lowering of symmetry changes the total energy?

[katrusiat]

Dear Stefaan and Andrei,

To check whether not big enough RKM could cause the difference in the total energy, I made a test calculation of TiOCl system. In the high symmetry (59_Pmmn) unit cell, there are 2 equivalent Ti atoms, which I made inequivalent to generate a low symmetry unit cell (25 Pmm2). With -ec 0.00001 and RKM = 7, the energies were:

HS = -5562.790746
LS = -5562.790740

and with RKM = 6,

HS = -5562.726158
LS =  5562.726155

I think this is enough to say that in this case the energy does not depend on the space group at all.

This case is different from that of Cs2CuCl4 I quoted before in that now for HS and LS I had equal number of k-points in IBZ, whereas for {1} and {2} of Cs2CuCl4 the number of k-points in IBZ was 120 and 192.
I would conclude therefore that Andrei is right, and the results for Cs2CuCl4 are not accurate because k-mesh is not dense enough.

Thank you very much for usefull suggestions,

Kateryna Foyevtsova





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