TS Optimization

Optimizing transition states with VASP is usually done with the dimer method. To use this method, the VTST for VASP program addon need to be installed in your VASP version. A usual way to obtain a TS for a given reaction is to first optimize the reaction path with the NEB method and then taking the image with the highest energy (if the climbing image NEB algorithm is used).

For a TS optimization, add the following keywords to a standard INCAR file (see here for basics):

• ICHAIN = 2 Activates the dimer method
• IBRION = 3 MD with zero time step
• POTIM = 0 Zero time step, ions are not moved
• DdR = 0.005 The dimer separation, twice the distance between the image.
• DRotMax = 4 Number of rotation steps per dimer translation
• DFNMin = 0.01 Lower threshold for dimer rotation (rotational force)
• DFNMax = 1.0 Upper threshold for dimer rotation

Aside these settings, the POSCAR file needs to include the initial guess for the TS structure.

Further, a file named MODECAR must be generated. In this file, the initial direction along the imaginary mode, where the dimer is started, must be given. If a frequency calculation on the TS initial guess has been done beforehand, the OUTCAR file from this calculation must be opened. Go to the end of the file and search from below the section in which the normal modes of the vibrations are listed. Take the the mode with the largest imaginary frequency (or the one, which resembles best the reaction of interest, after looked at in gmolden, use the [split_freq](add link) program!) and copy the right part with the displacements, for example:

18 f/i=   15.881852 THz    99.788616 2PiTHz  529.761543 cm-1    65.682031 meV
           X         Y         Z           dx          dy          dz
    9.057219 12.556697 15.441843     0.016824    0.007018   -0.000929
   11.033089 14.113899 16.148915    -0.021587    0.001532   -0.008405
   11.070764 13.777563 14.308820    -0.021653    0.001840    0.004560
   11.395509 12.363126 15.525165     0.230301    0.074750   -0.007783
   11.124066 13.414659 15.327396    -0.260907   -0.044059    0.002170
   13.334577 13.413904 15.372707     0.045110   -0.003492    0.001647

The last three columns without headers must be copied to the MODECAR file, such that it has the following content:

  0.016824    0.007018   -0.000929
 -0.021587    0.001532   -0.008405
 -0.021653    0.001840    0.004560
  0.230301    0.074750   -0.007783
 -0.260907   -0.044059    0.002170
  0.045110   -0.003492    0.001647

Now, the dimer TS optimization can be started. The resulting CONTCAR file might again be tested with an additional frequency calculation.