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) -------------- next part -------------- An HTML attachment was scrubbed... URL: http://www.democritos.it/pipermail/pw_forum/attachments/20120507/96240df5/attachment-0001.htm