Raman Spectrum of carbon chains: How to identify the vibrational modes


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Hello everyone, so I am using the following script to calculate raman spectra:

start c2h2
title "TDDFT c2h2 BP86/aug-cc-pVDZ"

geometry units angstroms
load monomer.xyz
end

basis spherical
H library 6-31G*
C library 6-31G*
end

driver
clear
maxiter 500
xyz test
end
dft
xc b3lyp 
convergence energy 1d-8 nolevelshifting
direct
TOLERANCES tight
ITERATIONS 5000
end

hessian
thresh 1d-8
profile
end

property
response  1 8.8559E-2
damping 0.007
end

raman
 normal
end

task dft optimize
task dft raman

First I do a calculation for acetylene, and i get the following results from 'c2h2.normal':
  1. freq [ 1/cm] S [Ang**4/amu]


 1   644.63        0.0087
2 679.18 0.0076
3 792.83 0.0001
4 2146.53 8.0917
5 3506.05 0.0007
6 3645.57 37.8189

Then carrying out the same calculation for C4H4, I get the following:
  1. freq [ 1/cm] S [Ang**4/amu]


 1   216.77        2.3873
2 305.29 0.7775
3 548.05 0.8095
4 643.94 1.7042
5 677.47 0.5218
6 700.29 6.8906
7 893.88 0.1890
8 928.98 0.5379
9 1033.68 0.1554
10 1134.09 1.5802
11 1357.84 16.9774
12 1504.93 31.2415
13 1715.33 17.1094
14 2269.08 38.4659
15 3293.55 19.5975
16 3313.05 78.1132
17 3449.94 44.9757
18 3578.50 24.6781

My question is , how do you identify what mode corresponds to what sort of movement. If I wanted to only look at the raman active C-C stretching modes, how could I do this?

Thank you.

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SOLUTION
One can identofy the vibrational modes, for example, by using the software package avogadro. If you run a nwchem calculation as such, let's sya:

   nwchem ir_frequencies.nw > slurm.out

the results will be stored in slurm.out, so now you can use avogadro as such:
   
avogadro input_structure.xyz slurm.out

And the vibrational frequnecies will be automatically displayed. Note that the file has to have a 'out' extension otherwise avogadro will not read it


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