[Pw_forum] convergence of phonon

[Eduardo Ariel Menendez P]

Dear colleagues,

I am doing a phonon calculation for a rather large system, with 12 atoms
in the unit cell and is metallic (Ti_3Si_C). The phonon calculations are
long. I started one 15 days ago and  for most q-points it is still running.
I need some advice to perform the convergence check.

1) Which parameters are relevant for a phonon calculation?
I guess: number of k-points,degauss,ecut,ecutrho
anyone else?

2) What output should be checked for convergence in previous SCF
calculations: the forces, or the energy is enough?
?What convergence of forces or energy can be considered safe for a phonon
calculation? Considering the following data, which ecut can I use?

ecut(Ry)  total_E(Ry) 	P(kbar)  Force(Ry/au)	Force(eV/A)

30	 -761.90543963 	-98.68 	-0.00675419 	-0.173648
40 	 -761.96493584 	-92.26 	-0.00574380 	-0.147672
50 	 -761.97745068 	-80.15 	-0.00621080 	-0.159678
60 	 -761.97911012 	-80.46 	-0.00611748 	-0.157279
70 	 -761.98131155 	-80.07 	-0.00611850 	-0.157305
80 	 -761.98190159 	-79.20 	-0.00615275 	-0.158186
90 	 -761.98206846 	-79.33 	-0.00613964 	-0.157849
100 	 -761.98234169 	-79.42 	-0.00613564 	-0.157746

Above, Force is taken as the z force component on a specific atom wich is
not at the theoretical equilibrium position. P(ressure)
is negative as I am using the experimental lattice constant
(should I relax the cell volume?).

Thank you


Eduardo A. Menendez Proupin
Department of Physics
Faculty of Science
University of Chile
Las Palmeras 3425
Ñuñoa, Santiago
Chile
Phone: 56+2+678 74 11
http://fisica.ciencias.uchile.cl/~emenendez/