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Dear all,

I have a question about the elastic constants calculation in VASP 5.2. VASP makes uniaxial strains, that is, deformations are done in Cartesian coordinates. If the system is hexagonal, I expect C11 and C22 should not be the same (symmetry are along a and b, not x and y), so this is not the same as the traditional Cij definition. How can I use these numbers to calculate the Bulk, Young's, Shear moduli?
Thanks.

Even the system is hex, the elastic constants are still defined in Cartensian coordinates. You need to figure out that under uniaxial loading which elastic constant comes in play.
There are more independent elastic constants in hex than cubic, which means you need more calculations to get the elastic constants. Once you get them, you need to use a certain homogenization method to homogenize them into shear and Young's moduli. Regarding the bulk modulus, it is usually done by the equation of states, not by calculating the elastic constants.
<span class='smallblacktext'>[ Edited Tue Oct 19 2010, 11:57AM ]</span>