[Pw_forum] cutoff convergence of sapphire for elastic properties

[Jörg Buchwald]

Hi,
i tested a relaxed sapphire structure  using a 10x10x10 k point set with
a cutoff energy up to 250 Ry. Even at this Energy, a change of +/- 5 Ry
in the cutoff lead to stresses of the order of 1 kbar.
I used for Al and O pbe-n-van US pseudo potentials from the pseudo
folder of quantum espresso. But also pbe-sp-van for Al and rrkjus for
oxygen led to the same problem. 
I don't have a clue how high the cutoff must be, but 250 Ry seems to be
far too high. Is that a problem of the pseudopotentials or does
it have something to do with physical structure of sapphire?
Thx for any hints,
Jörg Buchwald

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