Hi Swaroop,

I don't know what software package you are using. Result output style can vary software to software. In short, if your optimized structure is either a local minima or global minima, all of the normal modes of vibrations will be real and not imaginary. If your structure is not a minima (either global or local) rather a saddle point (like Transition state) one or more of the normal modes will be imaginary (for transition state only one normal modes of vibrations will be imaginary). This way you can separate between minima and saddle points.

However, to best of my knowledge it is not possible to separate local and global minima by observing the normal modes (real or imaginary) because for both of them all the normal modes of vibrations will be real. If you want to find out local and global minima and distinguish them, you need to scan through all minima by means of some automated algorithm like basin hopping or particle swarm based optimization in combination with QM software like VASP or Gaussian.

PS. Some software like Gaussian do report imaginary freq as negative number e.g -450.31 and not as 450.31i

Thank you Dr. Turbasu

Exactly as nicely stated above by Turbasu, an imaginary frequency is indication of a saddle point. At minima whether they are local or global, all frequencies should be real. You would have to to scan the whole potential energy surface...

If you are using vasp, the you have to add some additional command lines in the INCAR file such as NFREE=2 or 3, POTIM=0.015, IBRION =5. For this I will suggest you to go through https://cms.mpi.univie.ac.at/wiki/index.php/Vibrational_frequencies_of_CO_on_Ni_111_surface#INCAR.

After running this INCAR for your system, you should get an OUTCAR file. In the linux terminal, all you have to type is " grep cm-1 OUTCAR".

This will display you something like this :

1 f = 18.436656 THz 115.840927 2PiTHz 614.980633 cm-1 76.247908 meV

2 f = 17.522974 THz 110.100091 2PiTHz 584.503472 cm-1 72.469220 meV

3 f = 9.947358 THz 62.501091 2PiTHz 331.808122 cm-1 41.138979 meV

4 f = 7.125050 THz 44.768007 2PiTHz 237.666067 cm-1 29.466847 meV

5 f = 6.485318 THz 40.748455 2PiTHz 216.326916 cm-1 26.821129 meV

6 f = 5.226277 THz 32.837668 2PiTHz 174.329836 cm-1 21.614153 meV

7 f = 4.308256 THz 27.069569 2PiTHz 143.707937 cm-1 17.817520 meV

8 f = 3.202253 THz 20.120349 2PiTHz 106.815662 cm-1 13.243459 meV

9 f/i= 1.344285 THz 8.446394 2PiTHz 44.840530 cm-1 5.559519 meV

In this output, 1 f , 2 f represents normal mode of vibration, and f/i represents imaginary mode of vibration.