[Pw_forum] cutoff convergence of sapphire for elastic properties

[Nicola Marzari]

On 08/05/2012 18:42, Jörg Buchwald wrote:
> But to get the elastic constants in the elastic regime, i would
> like then apply strains of serveral per mill, which is of the same
> order of magnitude, i.e. also corresponds to changes of the stress
> tensor of the order of 1 kbar, which means that errors in the kbar
> range would be too high.


Hi Joerg,


relative quantities (in this case, difference in stress upon change
in strain) typically converge much faster than absolute quantities (the
stress for a given strain), so you should be fine. Worth checking.

> An alternative could be the calculation of the elastic constants using
> the second derivative of the energy. But this won't work for big
> supercells due to the computation time and the number of measuring
> points needed.

I think they would be comparable - for one elastic constant
using the stress tensor I would probably do three calculations -
one around equilibrium, and plus or minus 0.5% that . Using the energy,
maybe five? +1 +0.5 0 -.5 -1?


			nicola

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Prof Nicola Marzari, Chair of Theory and Simulation of Materials, EPFL