### What is considered a low frequency mode?

Posted:

**Tue Jan 12, 2021 8:25 pm**Hi All,

Recently I have been performing NEB calculations to search the transition state between C2H2 to C2H3 adsorbed over a surface. Then I preformed a frequency calculation (IBRION = 5, EDIFFG = -0.02, EDIFF = 1e-7, POTIM=0.015) to ensure that the transition state found is indeed a transition state by freezing all atoms in the surface and only allowing the adsorbate atoms to move.

As a result, I noticed there were two imaginary frequencies. I searched through this forum and the web and learned that if my system can translate or rotate, these low modes should be disregarded. I assumed because the molecules were adsorbed over a surface, it could be possible that the molecules can translate across the surface and therefore the second imaginary frequency calculated was due to a translation frequency.

However, my question is, what is considered a low frequency mode? Is there a "frequency cut-off" in which any imaginary frequency higher than a certain energy is considered to be too high to be a translational frequency mode and the NEB calculation needs to be re-done? (i.e. is an imaginary frequency with energy of 11 meV [89.2 cm-1] considered to be a high or low frequency mode?)

Thank you in advance!

Recently I have been performing NEB calculations to search the transition state between C2H2 to C2H3 adsorbed over a surface. Then I preformed a frequency calculation (IBRION = 5, EDIFFG = -0.02, EDIFF = 1e-7, POTIM=0.015) to ensure that the transition state found is indeed a transition state by freezing all atoms in the surface and only allowing the adsorbate atoms to move.

As a result, I noticed there were two imaginary frequencies. I searched through this forum and the web and learned that if my system can translate or rotate, these low modes should be disregarded. I assumed because the molecules were adsorbed over a surface, it could be possible that the molecules can translate across the surface and therefore the second imaginary frequency calculated was due to a translation frequency.

However, my question is, what is considered a low frequency mode? Is there a "frequency cut-off" in which any imaginary frequency higher than a certain energy is considered to be too high to be a translational frequency mode and the NEB calculation needs to be re-done? (i.e. is an imaginary frequency with energy of 11 meV [89.2 cm-1] considered to be a high or low frequency mode?)

Thank you in advance!