Calculate IR Intensities with VASP (WIP)

This tutorial covers how to calculate IR spectra using VASP.

1. Relax the structure.

2. Conduct a phonon calculation.

It is important to set the following INCAR tags:

• EDIFF: should be no larger than 10^-6
• IBRION: set to 5 (do not use symmetry to reduce the number of directions) or 6 (use symmetry to reduce the number of directions)
• LEPSILON: must be set to .TRUE. in order to calculate the Born effective field tensor
• NFREE: set to either 2 or 4; determines how many displacements in each direction are used for the finite differences
• NSW: must be set to exactly 1 in order for the finite differences step to be calculated
• POTIM: should be set to a very small number; the default is 0.015 but a valid number can be obtained by progressively increasing from 0.015 and comparing with the results from a DFPT phonon calculation

Additionally, the calculation must be a gamma-point only calculation since as of 4/17/2024, VASP DFPT only works for \(q = \Gamma\). Also note that, depending on how you run VASP, a separate executable may be selected based on the k-points chosen for the calculation. DFPT calculations are not supported with the vasp_gam executable.

Resources

VASP Wiki for IBRION INCAR Tag Stack Exchange Question with Instructions