[Pw_forum] scan the lattice constant-seems not a closed question

Nicola Marzari marzari at MIT.EDU
Mon Sep 12 17:11:19 CEST 2005

> Nicola said:
> == You see in fact your energy drop going to the
> left.==
> But it seems to me the energy drop going to the right.

Right (on two counts), I actually meant saying "going to the right",
i.e. towards larger lattice parameters, i.e. more plane waves
in the set.

> And furthermore, the example figures given by Nicola
> about the convergence test of Diamond and Graphite a
> bit puzzled me. As is put by Nicola and basically I
> believe so, more complet basis set will give lower
> energy. But in the example figures, especially the
> Diamond case, the energies obtained at the same k
> points seem to go higher with the increasing cutoffs. 

Wait ! The energy drops very significantly, and the curves
become smoother and smoother, as the plane-wave cutoff
increases from 24 to 26 to 28 to 30 Ry - i.e. going from black to
blue. This is the clear result of enlarging your basis set.

The very subtle differences in going from left to right, for
a given colour (i.e. given plane wave cutoff) have to do with
finer and finer charge denisty grids, i.e. more accurate
representations of the ultrasoft augmentation charges.


Prof Nicola Marzari   Department of Materials Science and Engineering
13-5066   MIT   77 Massachusetts Avenue   Cambridge MA 02139-4307 USA
tel 617.4522758  fax 617.2586534  marzari at mit.edu  http://nnn.mit.edu

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