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

[Casey Brock]

Hi Pietro,

Using v5.1 or v6.4, I get the correct answer. Perhaps there was a bug in v6.3MaX, or maybe there is just a problem with my build. In any case, I have a working build of 6.4 now so problem solved.

Thanks!
Casey

________________________________
From: users <users-bounces at lists.quantum-espresso.org> on behalf of Casey Brock <casey.brock at hotmail.com>
Sent: Wednesday, March 27, 2019 12:44 PM
To: Quantum Espresso users Forum
Subject: Re: [QE-users] incorrect LO-TO splitting for CaO and MgO

I am using v.6.3MaX

Casey
________________________________
From: users <users-bounces at lists.quantum-espresso.org> on behalf of Pietro Davide Delugas <pdelugas at sissa.it>
Sent: Wednesday, March 27, 2019 11:20 AM
To: users at lists.quantum-espresso.org
Subject: Re: [QE-users] incorrect LO-TO splitting for CaO and MgO

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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