[QE-users] Convergence and the "dual": norm-conserving vs PAW

[Christoph Wolf]

Dear all,

I am afraid that this is a very basic question but I will ask it anyway in
the hopes of some pointers. I have recently studied the convergence
behavior of a set (Mg and O) pseudos with respect to the phonon frequencies
and I encounter a behavior that I quite not understand.

I study the convergence of total energy and the highest phonon frequency at
q=(0.5,0.5,0.5). I vary the dual=ecutrho/ecutwfc=2,4,8.

for the norm-conserving pseudos (standard dual is 4 here) energy converges
(monotonously) at a ecutwfc=50 irrespective of the dual. however the phonon
frequency only converges (strongly non-monotonously, i.e. in a zig-zag
pattern) if (i) ecutwfc=100 and dual 8 OR (ii)ecutwfc=200 and dual 4

for the PSLibrary 1.0.0 pseudos the total energy converges (monotonously)
at ecutwfc=50 for all duals but interestingly so does the phonon frequency
(in excellent agreement with the converged norm.-cons.!). Varying the dual
from 2 to 8 leaves the phonon frequencies virtually unchanged. the
suggested hardest cutoff for the pseudos (from the file) is Mg: 97/398 -
higher than what I found.

Now I have read that for phonons and US/PAW often a dual of 8-12 is advised
(I think the example is for metals not an insulator as MgO) but I was
curious if there is any "fool proof" method to ensure the convergence
whilst not risking falling in a "local minima" of a phonon frequency, for
example?

Thanks for reading all the way down to hear, your help is greatly
appreciated!

Best,
Chris

-- 
Postdoctoral Researcher
Center for Quantum Nanoscience, Institute for Basic Science
Ewha Womans University, Seoul, South Korea
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