[QE-users] incorrect LO-TO splitting for CaO and MgO

[Pietro Davide Delugas]

Hi Casey
using your inputs cut and paste I get the right values.

# mode   [cm-1]    [THz]      IR
     1     -0.00   -0.0000    0.0000
     2     -0.00   -0.0000    0.0000
     3      0.00    0.0000    0.0000
     4    291.10    8.7268   11.3884
     5    291.10    8.7268   11.3884
     6    569.57   17.0753   11.3884

What version are you using ?


On 03/27/2019 03:19 PM, Casey Brock wrote:
> I am using pw.x -> ph.x -> dynmat.x to get the LO and TO phonon 
> frequencies at gamma for rock-salt CaO. The value for the LO phonon I 
> am getting differs from experiment and other DFPT calculations in the 
> literature by 5 THz (167 cm^-1). Perhaps this is a problem with 
> Quantum Espresso, but it’s very possible I’m doing something wrong. 
> I’ve included results and input files below, and any advice would be 
> greatly appreciated.
>
> The TO and LO frequencies in THz are given below. Notice that Quantum 
> Espresso using DFPT gives a value that is too low.
> experiment: 8.9, 17.1
> frozen phonon (Phonopy + Quantum Espresso): 8.71, 17.1
> DFPT (Quantum Espresso, pw.x->ph.x->dynmat.x):  8.53, *12.2*
> *
> *
> If I use Phonopy (frozen phonon technique) combined with Quantum 
> Espresso (pw.x and ph.x) I get the correct splitting at gamma. I have 
> tried the same calculation with MgO and I see the same problem, where 
> the frozen phonon technique works while DFPT does not. Here are some 
> things I have tried: different PAW pseudopotentials, increasing 
> ecutwfc and ecutrho, increasing k-points, reducing conv_thr, and 
> reducing tr2_ph.
>
> Thanks in advance,
> Casey Brock
>
> input to pw.x
> -------------------------------------------
> &control
>     calculation='scf'
>     restart_mode='from_scratch'
>     prefix='cao'
>     pseudo_dir = './'
>     outdir='./output/'
>     tprnfor = .true.
>     tstress = .true.
> /
> &system
>     ibrav = 0
>     nat = 2
>     ntyp = 2
>     ecutwfc = 70.0
>     ecutrho = 280.0
> /
> &electrons
>    conv_thr =  1.0d-10
>    mixing_beta = 0.7
> /
> K_POINTS {automatic}
>  12 12 12 0 0 0
>
> CELL_PARAMETERS bohr
>  4.45 4.45 0
>  0 4.45 4.45
>  4.45 0 4.45
> ATOMIC_SPECIES
>  O  16.00 O.pz-n-kjpaw_psl.0.1.UPF
>  Ca 40.08 Ca.pz-spn-kjpaw_psl.1.0.0.UPF
> ATOMIC_POSITIONS crystal
> Ca  0.0  0.0  0.0
> O   0.5  0.5  0.5
>
> input to ph.x
> ---------------------------------------
> Phonon for CaO
> &inputph
>  prefix='cao',
>  outdir='./output/',
>  fildyn='dyn.G'
>  epsil = .true.
>  tr2_ph=1.0d-14,
> /
> 0 0 0
>
> input to dynmat.x
> ---------------------------------------
> &input
>  fildyn = 'dyn.G'
>  asr = 'simple'
>  q(1)=0.0, q(2)=0.0, q(3)=1.0
> /
>
>
>
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