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

[Nicola Marzari]

> 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.

			nicola



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