I obtained the transition state (TS) using the CI-NEB method with 5 intermediate images. The image corresponding to the TS has an imaginary frequency of ~350-400 cm⁻¹ and the highest energy along the pathway. However, when I interpolated more images between the converged 5 images along the reaction path, I noticed that the preceding two images (lower in energy by ~0.04 eV and ~0.2 eV, respectively than the TS) have significantly higher imaginary frequencies (~890 and 1100 cm⁻¹).
When I re-ran CI-NEB with the coordinates of these preceding images (as potential TS candidates), the optimization converged back to the same TS with ~350 cm⁻¹ as the imaginary frequency. I also tried connecting the preceding image and the TS through CI-NEB, but no barrier was observed.
My questions are:
How do I ensure that the TS is correctly identified?
Are there any specific changes needed in my VASP input file to improve convergence or accuracy? Here are my current settings:
LSCALAPACK= .FALSE.
NPAR=1
LREAL = Auto
ISYM= 0
ALGO= FAST
ENMAX=800
EDIFF= 1E-6
EDIFFG= -0.01
POTIM= 0.015
IBRION= 5
NFREE= 2
ISIF= 2
ISPIN= 2
ISMEAR= 2
SIGMA= 0.2
ADDGRID= .TRUE.
LCHARG= .FALSE.
LWAVE= .FALSE.
IVDW= 12
The PES does not seem flat, so I am puzzled about why these preceding images show higher imaginary frequencies. Any guidance or suggestions would be highly appreciated!
How to Handle TS with Lower Imaginary Frequency than Preceding NEB Images?
Moderator: moderators
Re: How to Handle TS with Lower Imaginary Frequency than Preceding NEB Images?
I don't see any problem with configurations away from the TS having higher or lower negative curvatures. I can think of situations where either case will occur.
Re: How to Handle TS with Lower Imaginary Frequency than Preceding NEB Images?
Thank you for your response! Can I confidently report the TS obtained via CI-NEB with an imaginary frequency of ~300 cm⁻¹ as the TS, even though preceding images show higher imaginary frequencies (~800-1100 cm⁻¹)? Would an IRC calculation be necessary to further confirm the TS, or is this setup sufficient to validate my results?
Re: How to Handle TS with Lower Imaginary Frequency than Preceding NEB Images?
I just don't understand where this concern is coming from. A first order saddle is defined as a point on the potential with zero force and one negative mode. There can be lots of other points on the potential surface with lower negative modes - but who cares?
To your other question: you can calculate the IRC if you want to but it is not 'necessary' to confirm a saddle. Again, a first order saddle is defined as a point on the potential with zero force and one negative mode.
To your other question: you can calculate the IRC if you want to but it is not 'necessary' to confirm a saddle. Again, a first order saddle is defined as a point on the potential with zero force and one negative mode.
Re: How to Handle TS with Lower Imaginary Frequency than Preceding NEB Images?
Thank you for the clarification. I now understand that this is not a problem as long as the TS itself satisfies the criteria of a first-order saddle point.