Imaginary modes

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Imaginary modes

Postby Tobias Kraemer » Mon Nov 13, 2017 12:39 pm

Hi all,

Following a geometry optimisation of my structure (organometallic species with 324 atoms containing Rh, P, C, H, B, F) I performed a vibrational analysis. The calculation revealed the presence of 4 small imaginary modes (8-20 cm-1). I would like to ask whether these should be discarded as noise or whether they are real imaginary modes that should be reoptimised. I am generally trying to get rid of any imaginary mode in the structure, but sometimes this is difficult and one could go on forever doing so. Since I used fairly default settings for the geometry optimisation, with EDIFF=10-7, I wondered if there was a more robust protocol to generate tighter convergence during the optimisation runs. I have tried the EDIFFG tag, but with little success (more imaginary modes). I am not sure I have done this right though. Is there a way of tightening the convergence up in a consistent way? I know that I should also have an eye on the SCF convergence, since the gradients critically depend on the energy.

I would much appreciate any comments on geometry optimisations and what to do in cases when one is left with a number of small imaginary modes. Ignore? Reoptimise?

Thanks in advance (see INCARs below)

Tobias


INCAR (opt)
General:
SYSTEM = Rh-dcype-hexane
! if enough K-points are present -5 should be used
ISMEAR = 0 ! 0: Gaussian -> semicond/insulator ; 1-N: MP; -5: Tetra+Blochl -> metal/very accurate energy/forces
SIGMA = 0.05
EDIFF = 1.0E-7
PREC = Normal ! to make ROPT=2.0E-4 for LREAL=Auto
LREAL = Auto ! cheaper for large cell's we are using
ENCUT = 600 !
ISYM = 2 # use symmetry as done for PAW PP

NELMIN = 8
LWAVE = .FALSE.
LCHARG = .FALSE.
! LVTOT = .TRUE. ! Write LOCPOT file for Toon
! LVHAR = .TRUE. ! but LOCPOT without exchange and correlation contribution for forcefields Toon
VOSKOWN = 1 ! important for GGA (PW91) for interpolation of XC...since we only use PBE=switch on (LDA: VOSKOWN=0)
LASPH = .TRUE. ! For VASP.5.X the aspherical contributions are properly accounted for in the Kohn-Sham potential
! as well. This is essential for accurate total energies and band structure calculations for f-elements
! (e.g. ceria), all 3d-elements (transition metal oxides), and magnetic atoms in the 2nd row (B-F atom),
! in particular if LDA+U or hybrid functionals or meta-GGAs are used, since these functionals often result
! in aspherical charge densities.

Van der Waals Interaction (vasp 5.3.3 patched verion):
IVDW = 12 ! switches between 0:off, 1: DFT-D3 and 2: TS-VDW (default=1)

dynamic:
IBRION = 2 ! -1: Fix atoms; 0: MD; 2: ConjGrad relax; 44: improved dimer method
NSW = 500 ! Number ionic steps
ISIF = 0 # relax ions, no change cell shape, no change cell volume (volume changes require ENCUT*1.3)
POTIM = 0.2

parallel:
LPLANE = .TRUE.
NPAR = 16

INCAR (phon)
General:
SYSTEM = Rh-dcype-hexadiene
! if enough K-points are present -5 should be used
ISMEAR = 0 ! 0: Gaussian -> semicond/insulator ; 1-N: MP; -5: Tetra+Blochl -> metal/very accurate energy/forces
SIGMA = 0.05
EDIFF = 1.0E-8
PREC = Normal ! to make ROPT=2.0E-4 for LREAL=Auto
LREAL = Auto ! cheaper for large cell's we are using
ENCUT = 600 !
ISYM = 2 # use symmetry as done for PAW PP

NELMIN = 8
LWAVE = .FALSE.
LCHARG = .FALSE.
! LVTOT = .TRUE. ! Write LOCPOT file for Toon
! LVHAR = .TRUE. ! but LOCPOT without exchange and correlation contribution for forcefields Toon
VOSKOWN = 1 ! important for GGA (PW91) for interpolation of XC...since we only use PBE=switch on (LDA: VOSKOWN=0)
LASPH = .TRUE. ! For VASP.5.X the aspherical contributions are properly accounted for in the Kohn-Sham potential
! as well. This is essential for accurate total energies and band structure calculations for f-elements
! (e.g. ceria), all 3d-elements (transition metal oxides), and magnetic atoms in the 2nd row (B-F atom),
! in particular if LDA+U or hybrid functionals or meta-GGAs are used, since these functionals often result
! in aspherical charge densities.

Van der Waals Interaction (vasp 5.3.3 patched verion):
IVDW = 12 ! switches between 0:off, 1: DFT-D3 and 2: TS-VDW (default=1)

dynamic:
IBRION = 6 ! -1: Fix atoms; 0: MD; 2: ConjGrad relax; 44: improved dimer method
NSW = 1 ! Number electronic steps
ISIF = 0 # relax ions, no change cell shape, no change cell volume (volume changes require ENCUT*1.3)
NFREE = 2
POTIM = 0.015

parallel:
LPLANE = .TRUE.
# NPAR = 24
# NCORE = 24
Tobias Kraemer
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