[Pw_forum] about elastic constant
Michael Mehl
rcjhawk at gmail.com
Thu Jul 16 22:57:38 CEST 2009
Bipul Rakshit wrote:
> Dear Michael Mehl,
> I have read the two papers
> http://cst-www.nrl.navy.mil/users/mehl/papers/cij453.pdf
>
> and
>
> Phys. Rev. B 47, 2493 (1993)
> ,
> But my question is, they have given only two equation, and our unknown
> quantity are 3 (C11, C12, and C44). So can we find three unknown
> quantity from two equations acuratelly.
C44 is computed directly.
C' = C11-C12, or maybe 1/2 (C11-C12).
For a cubic crystal, B = (C11 + 2 C12)/3.
That's three equations for three unknowns.
C44 and C' are computed as outlined in Wang's paper, my papers, and
elsewhere. The bulk modulus is obtained from
B = V E''(V),
and you can compute the second derivative of the energy by finding the
energy at several volumes and either using one of the many
equation-of-state fitting codes available, or via numerical differentiation.
--
Michael J. Mehl, Naval Research Laboratory, Washington DC
On Sabbatical at Duke University until 31 July 2009
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