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<p>you can locate the equilibrium geometry with vc_relax and then
manually apply homogeneous deformation, relaxing the INTERNAL
degrees of freedom and obtaining the elastic constant from the
stress strain relationship. you will need to apply +/-
deformations of various strength and extract the linear term in
the variation of the stress tensor. Traceless deformations are
preferable because they don't change the average number of
planewaves. make sure your cutoff is converged for a stress
calculation. <br>
</p>
<p>If the system has some symmetry this helps reducing the number of
calculations needed.</p>
<p>For instance for cubic symmetry two deformations are enough: an
e1 deformation (voigt notation
<a class="moz-txt-link-freetext" href="https://en.wikipedia.org/wiki/Voigt_notation">https://en.wikipedia.org/wiki/Voigt_notation</a>) gives C11 and C12
and e4 gives C44<br>
</p>
<p>for a generic crystal you need six deformations (e1,..,e6) to get
the 21 independent elastic constants.</p>
<p>stefano<br>
</p>
<div class="moz-cite-prefix">On 29/03/25 15:03, Jonathan Konyi
Kakulu wrote:<br>
</div>
<blockquote type="cite" cite="mid:AM0PR0102MB3524FD29ECB1ACA6EE4869EACDA32@AM0PR0102MB3524.eurprd01.prod.exchangelabs.com">
<div dir="auto">I have get bulk modulus with ev.x . I want to know
how to calculate the C11, C12 and the C44 constant with PWSCF </div>
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