[QE-users] Phonons of metal/insulator interfaces: convergence problems

[Christoph Wolf]

Dear all,

I am curious if this is a common problem: when calculating the phonons of
an insulator and a metal I face no problems in slab geometry. But when both
are put together there will always be a q-point where convergence cannot be
reached, for example:

*     Computing dynamical matrix for *
*                    q = (   0.0000000   0.1666667   0.0000000 )*
*       2 Sym.Ops. (no q -> -q+G )*

*     G cutoff =  609.4453  (  20928 G-vectors)     FFT grid: ( 50, 50,400)*

*     number of k points=   144  gaussian smearing, width (Ry)=  0.0100*


*      iter #  15 total cpu time : 24287.3 secs   av.it <http://av.it>.:
13.0*
*      thresh= 4.091E-07 alpha_mix =  0.700 |ddv_scf|^2 =  8.763E-11*
*...*

*      iter #  59 total cpu time : 27386.7 secs   av.it <http://av.it>.:
11.2*
*      thresh= 5.160E-05 alpha_mix =  0.700 |ddv_scf|^2 =  1.219E-06*

*..*
*      iter # 100 total cpu time : 30214.5 secs   av.it <http://av.it>.:
10.8*
*      thresh= 1.000E-02 alpha_mix =  0.700 |ddv_scf|^2 =  4.154E-02*

*     End of self-consistent calculation*

*     No convergence has been achieved*

Due to the oscillating behavior of ddv_scf I doubt that increasing the
iterations will in the end work.

I tried increasing k points and cutoff well beyond SCF convergence (SCF
shows convergence at 12x12x1 and ecutwfc~80 Ry but I went up to 36x36x1 and
120 Ry.)

&inputph
  prefix   = 'mgo',
  trans    = .true.
  epsil    = .false.,
  fildyn   = 'mgo.dyn',
  ldisp    = .true.
  fildvscf = 'dvscf'
  amass(1) = 107.86820
  amass(2) = 24.30500
  amass(3) = 15.99990
  outdir='./',
  nq1=6,  nq2=6,  nq3=1,
  tr2_ph   =  1.0d-12,
 /

The system is MgO stacked on top of Ag. The pseudos are NC from the ONCV
library and work very well for the "pure" Ag and MgO separately.

Any hint is very welcome!

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