[DFTB-Plus-User] Treatment of Imaginary Frequencies

[rsaha at icredd.hokudai.ac.jp]

Hello everyone,

I have a query about the small imaginary frequencies in the DFTB 
computations.

I am doing frequency (SCCTolerance = 1e-8 & RemoveTranslation = Yes) 
calculations after optimisation of the unit cells. The unit cells have ~
600 atoms. There are a few imaginary frequencies (as shown below for two 
different systems) that occurred which are very small.  

 Vibrational modes (cm-1):
    1   -1.46
    2   -0.00
    3   -0.00
    4    0.00
    5    7.40
    6    7.41

OR

 Vibrational modes (cm-1):
    1   -2.09
    2   -1.02
    3   -0.00
    4    0.00
    5    0.00
    6    4.94

In this condition, should I need to remove these imaginary frequencies?

Thank you.
Regards
Ranajit