[Pw_forum] cutoff convergence of sapphire for elastic properties

[Mike Mehl]

To follow up on Nicola's point, the bulk modulus of sapphire is 240 GPa or
2400 kbar (http://www.mt-berlin.com/frames_cryst/descriptions/sapphire.htm).
 So a 1 kbar error corresponds to a very small change in volume.

If we use the quick and dirty Birch equation of state:

P(V) = 3/2 K0 [ (V0/V)^(7/3) - (V0/V)^(5/3)]

with K0 = 2400 kbar and ask what volume will produce a 1 kbar change in
pressure we get

delta V/V0 = +/- 0.0004

Considering the normal errors in DFT, it's not worth trying to converge the
stress to the 1 kbar accuracy you're trying to achieve.


On Mon, May 7, 2012 at 10:07 AM, Nicola Marzari<nicola.marzari at epfl.ch>
 wrote:

>
> How much does 1 kbar error translates into an error in lattice
> parameter? (keep atoms fixed, using relative coordinates, cutoff fixed,
> and expand celldm(1) by 0.3% - what's the change in stress? that change
> should be very well converged)
>
> --
Michael Mehl
US Naval Research Laboratory
Washington DC
(Home email)
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