Ruslan, This is already off the limits of DFT precision... I'd say it's just numerical noise. If the fluctuations were much larger (say, of the order of a few tens of eV), then I might be tempted to wonder about it. I would say your calculation is already very well converged at about 70-80 Angstrom. By email correspondence, a friend told me that this convergence problem could be greatly diminished by increasing the mesh cutoff, which really leads me to think this could be a spurious numerical issue, maybe a spurious dipole moment. Try doubling the mesh cutoff, or even multiplying it by three, and check out what the effect is on the numerical results. However, if you're using siesta 3.0, have you tested the feature of dipole correction for slabs? (This is actually a question driven by curiosity, since I have not tested it myself but would love to know what its behavior is like) It could save you some cutoff increase - wild guess.
Cheers, Marcos On Sun, Apr 18, 2010 at 3:54 PM, Руслан Жачук <[email protected]> wrote: > Dear Siesta users, > > I study silicon slab, centrosymmetric (no hydrogen). > Thickness is about 15 Ang. > Now I want to find such vacuum space that Etot is converged. > > > But it converges until a value of about Etot=-9252.4480 > (at Hvac=10 Ang), but with further increase of vacuum space Etot > increase/decrease > (fluctuates) around this value by 0.0005 eV (see the graph attached). > Is it normal? Why this happen? Why Etot does not converge > to some exact value as expected? > > Kind regrads > Ruslan Zhachuk >
