[DFTB-Plus-User] How to Remove Imaginary Frequencies in DFTB+

[rsaha at icredd.hokudai.ac.jp]

Hello Everyone,

Recently, I am trying to do frequency calculations (Second derivative + 
modes) using DFTB+18.2. The systems have a total of 722 atoms.

(1) After the calculation, I have found that there are some imaginary 
frequencies. I am considering modes from 7th and so on.
"""
 Vibrational modes (cm-1):
    1  -21.22
    2  -20.11
    3  -16.83
    4  -15.29
    5  -13.92
    6  -12.07
    7  -11.08
    8   -9.81
    9   -8.27
   10   -6.61
   11   -5.34
   12    2.29
   13    4.83
   14    6.89
   15    9.37
"""

Is there any general procedure to remove these imaginary frequencies?
I have used G16/Gview6 before which was very easy to work with imaginary 
frequencies. I want to know what to do with DFTB+ outputs and I guess it 
will not be straightforward as G16/Gview.

Another part, the imaginary frequencies (from 7th and so on) are very 
small (< 50cm-1). Is it required to remove them? Generally, I remove all 
imaginary frequencies in my calculations. Please let me know your 
opinion.


(2) In some part of the mode_**.xyz files, the XYZ coordiantes look like 
this,
"""
H  -44.267149 -6.948083  0.672858
H  -43.163722-10.245054  3.204615
C  -47.879462 -0.613343 11.689011
"""

As you can see there is no space between the x and y coordinate in the 
second line, which causes problems in visualising these modes with 
Chemcraft (VMD works well though). Is there any way to clean this format 
and make it readable using Chemcraft?

Thank you all for your time and reading.

Regards
Ranajit