[Pw_forum] some naive problems

[mbaris at metu.edu.tr]

Dear Paul,

If I may, I want to add some pointers

> Just to add to this general discussion...which I think is very useful...when
> I take up a new problem I write two scf scripts, one which increments the
> size of the Monkhorst-Pack (K_POINTS) net until the total energy "sort of"
> settles down (I say "sort of," because sometimes there are small
> oscillations when the net gets big.  Then I write another script which takes
> the "best" MP size increments the cut-off upward until I'm happy.  If I
> think I need to find new "equilibrium" lattice constants, I'll write a third
> script which varies these.  Then I use all this to start computing
> observables.  I don't think I've ever used the gamma option.

observables like the lattice parameter often settle much quicker 
compared to total energy, thus if you are looking for speed in your 
calculations, it is often better to find a minimized parameter set 
according to an observable. In some cases you may end up with problems 
other than speed using a "large" parameter set. I also have a script 
that minimizes the energy according to desired observable first using a 
golden search algorithm then spline interpolation.



>
> Re psuedopotentials:  I pretty much trust the ones from the PWscf library
> and/or from Vanderbilt.
>
Yes, but sometimes older pseuodos may lack full headers describing how 
the pseudo was created.

> On the "naïve" side.  Most of the systems I investigate are far from cubic
> symmetry, and I choose the number of K_POINTS in each direction
> proportionately to reflect the way the dimensions of the Brillouin zone vary
> inversely with those of the Wigner-Seitz cell.  What do others do?  Also,
> I've never found adding an "offset" to the MP net has a significant
> effect...when should I expect differently?

Same here. The offset is used to include special points in your MP set, 
especially handy if you are using a rather small and even set and you 
want to have the gamma point explicitly. You may also increase the 
number of k-points in an even set to approximate gamma, or add the 
gamma point "by hand".



Best,
Baris,

Osman Baris Malcioglu
PhD. Candidate, METU Physics
Ankara Turkey