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The manual says that the default is in z-direction (the 3rd vector). Also According to the literature the first meta-stable state in DFT+U collinear calculations for UO2, the fluorite structure should be distorted such that the c-axis becomes shorter than a,b and (a=b). It is claimed that the spin in c-direction causes this contraction.

I tried to reproduce this and I got everything correct except that the contraction occurred in a-direction (the first vector) rather than the c-direction. I did not change the defaults for the the magnetic moment direction. Any hint about why I get that.

I use VASP 4.6.31 and I start my calculations from an Anti-ferromagnetic guess as it should be for UO2.

Did you comile VASP with the NGXhalf instead of NGZhalf?

I did not compile it myself. Is there a way to check which was used in compilation from the output files?

sorry, but I fear I don't understand: the fluorite structure is cubic, and unless you impose some external field, strain,... in a certain direction, there is no principal axis which could be identified as c (with respect to what would you choose?)