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I am planning to use the finite displacement method for calculating phonon spectrum, and I have prepared the INCAR file below to try it on a transition metal trihalide monolayer (e.g., VI$_3$). I would be very grateful if you could please check it and tell me what I should add/omit, and if you recommend following this Phonopy tutorial.

INCAR:

    NPAR = 6
# Self Consistent Calculation step:
ISTART = 0
ICHARG = 2

#Electronic Relaxation:
ENCUT = 550 eV
EDIFF = 1.0E-8
PREC = Accurate
LREAL = .FALSE.
IALGO = 38
ADDGRID=.TRUE.
LCHARG = .FALSE.
LWAVE = .FALSE.

#Phonons
IBRION = -1

#Relaxation:
ISMEAR = 0
SIGMA = 0.05
NSW = 1
GGA_COMPAT = .FALSE.

# Magnetism:
ISPIN= 2
MAGMOM = 4*-5.0 4*5.0 24*0.6
LASPH = .TRUE.
LMAXMIX = 4
VOSKOWN = 1

#Mixer
AMIX     = 0.2
BMIX     = 0.00001
AMIX_MAG = 0.8
BMIX_MAG = 0.00001

I am a little bit confused about which IBRION tag to use (IBRION=-1 or IBRION=6)

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1 Answer 1

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Phonopy creates different POSCAR file in which atoms are perturbed, as you are running each POSCAR for NSW=1 or 0, IBRION=-1 will be correct choice. For NSW=1 or 0 , only one self consistency loop will run with no update in ion position , all IBRION tags are equivalent.

IBRION=-1: no update.

The ions are not moved, but NSW outer loops are performed. In each outer loop the electronic degrees of freedom are re-optimized (for NSW>0 this obviously does not make much sense, except for test purposes). If no ionic update is required use NSW=0 instead.

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  • $\begingroup$ What about the ISYM tag? should it be modified or not? $\endgroup$
    – Chi Kou
    Dec 13, 2021 at 10:54
  • 1
    $\begingroup$ @Chi Kou phonopy generates different POSTER based on symmetry of equilibrium structure, once we got different perturbed structure, we run SCF calculation. For SCF calculation, there is no need to remove symmetry. Keep ISYM to default $\endgroup$ Dec 13, 2021 at 14:54
  • $\begingroup$ Thank you very much! that was a very well explained answer. $\endgroup$
    – Chi Kou
    Dec 13, 2021 at 17:45

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