[Pw_forum] Wild phonons

[Miguel Mart=?US-ASCII?Q?=ED?=nez]

Dear colleagues,

I've been working on high pressure phonon branches of alkali metals
lately. Having all stuff calculated with PBE, I obviously wanted an Na
PBE pseudopotential. My choice this time was Axel Kohlmeyer's
Na.pbe-nsp-van.UPF.

The thing is, at 20 GPa, even with ecutwfc=60 Ry and ecutrho=600 Ry, on
a 26 26 26 monkhorst pack grid, a change of degauss from 0.02 to 0.015
makes the low frequency mode in the [110] branch unstable. I really
can't imagine a 26 26 26 grid being too coarse for a notable jump on a
subtle smearing change. At q= 0.1 0.1 0, the results are: 

degauss = 0.02 Ry
     omega( 1) =       0.187580 [THz] =       6.257034 [cm-1]
     omega( 2) =       1.178767 [THz] =      39.319706 [cm-1]
     omega( 3) =       2.672894 [THz] =      89.158739 [cm-1]

degauss = 0.015 Ry
     omega( 1) =      -0.165534 [THz] =      -5.521640 [cm-1]
     omega( 2) =       1.183868 [THz] =      39.489864 [cm-1]
     omega( 3) =       2.673752 [THz] =      89.187356 [cm-1]

This being Na bcc at 20 GPa, having checked that its enthalpy is lower
than fcc Na with that pseudopotential, I'm worried to see such a jump in
the mode related to C'. I know that I'm close to gamma, making
convergence hard. But on the other hand, both the energy cutoff and the
k-point mesh are pretty large. 

If anybody has any comments, I'd gladly recieve them. 

By the way, is it too crazy of an idea to calculate the elastic
constants from the phonon spectrum instead of directly deformating the cell?

PS: I attach the input, in case anybody wants them:

####### Scf input:#######
 &control
    calculation='scf'
    restart_mode='from_scratch',
    tstress = .true.
    tprnfor = .true.
    prefix='na',
    pseudo_dir = '/home/wmbmacam/pseudo/',
    outdir='/scratch/wmbmacam/phon/'
 /
 &system    
    ibrav=  3, celldm(1)= 6.282 , nat=  1, ntyp= 1,
    ecutwfc = 60.0, ecutrho= 600.0
    occupations='smearing', smearing='m-p', degauss= 0.020
 /
 &electrons
    conv_thr =  1.0d-8
    mixing_beta = 0.7
 /
ATOMIC_SPECIES
 Na  22.989768  Na.pbe-sp-van_ak.UPF
ATOMIC_POSITIONS
 Na 0.00 0.00 0.00 
K_POINTS {automatic}
 26 26 26   1 1 1


###### Nscf input #######
 &control
    calculation='phonon'
    restart_mode='from_scratch',
    prefix='na',
    pseudo_dir = '/home/wmbmacam/pseudo/',
    outdir='/scratch/wmbmacam/phon/'
 /
 &system
    ibrav=  3, celldm(1)= 6.282 , nat=  1, ntyp= 1,
    ecutwfc =60.0, ecutrho = 600.0
    occupations='smearing', smearing='m-p', degauss=0.020
 /
 &electrons
    conv_thr =  1.0d-8
 /
 &phonon
    xqq(1) = 0.10, xqq(2) = 0.10, xqq(3) = 0.00
 /
ATOMIC_SPECIES
 Na  22.989768  Na.pbe-sp-van_ak.UPF
ATOMIC_POSITIONS
 Na 0.00 0.00 0.00
K_POINTS {automatic}
 26 26 26   1 1 1

###### Phonon input ######
 &inputph
  tr2_ph=1.0d-16,
  prefix='na',
  ldisp=.false.,
  amass(1)=22.989768,
  outdir='/scratch/wmbmacam/phon/',
  fildyn='dyn.0.10,0.10,0.00',
 /
 0.10 0.10 0.00