Normal Modes and IR Spectrum - Trebonius91/utils4VASP GitHub Wiki

IR spectra of systems of interest can be obtained by calculating, mass-weighting and diagonalizing its Hessian matrix. Within VASP, only numerical Hessian matrices can be calculated by small elongations of the relevant degrees of freedom. It is important that the structure is a minimum or saddle point, else, the quadratic approximation breaks down and the frequencies are meaningless!

To calculate a Hessian, add the following keywords to a standard INCAR file (see here to set it up first):

  • IBRION = 5 Activates a numerical frequency calculation
  • POTIM = 0.015 The standard degree of freedom numerical elongation, should almost always work well.
  • LDIPOL = .TRUE. Activates the dipole moment corrections (for IR intensities!)
  • IDIPOL = 3/4 For surface slabs (z-axis = surface normal), this need to be set to 3, for isolated molecules in the gas phase, 4 must be set.
  • DIPOL = 0.5 0.5 0.5 A location within the unit cell (direct coordinates) for the dipol correction. The center (0.5..) is the default, but might lead to SCF convergence problems!. If this happens, change the location somewhat, e.g., to 0.4 0.6 0.3.
  • ISYM = -1 Switch off symmetry completely (will be broken anyways due to the small numerical elongations). This is required, if the calculation is ran on more than one CPU node, else strange errors (changing of k-points sets) will occur after the first single point!

For large systems such as adsorbates on metal surfaces, one should use selective dynamics and only activate (T T T) the atoms of the adsorbate in order to keep the calculation effort manageable! Only activated atoms will be elongated and part of the Hessian.

Frequency calculations cannot be restarted in VASP! Therefore, walltime limits on calculation clusters can be problematic if one wants to treat large systems. Our new program [split_freq](add link!) is able to split a calculation in an arbitrary number of parts that can be recombined with it as well, such that this problem disappears.

The same program can also be used to generate a molden file from which the IR spectrum and the normal modes can be visualized. If you have generated the file, open it with molden or gmolden (link). There, click on Norm. Modes in the upper right corner. An IR spectrum is generated, the widths of the bands can be altered by the user. Select the respective mode on the list to view it in the molecule window.