Dear Nilangshu.
Two suggestions to remove ifreqs:
1. Increase accuracy of the integration grid that is used for numerical integrations.
This is recommended for molecules containing lots of tetrahedral centers and for computing very low frequency modes of systems. Increasing grid accuracy is also useful for optimizations of larger molecules with many soft modes such as methyl rotations, making optimizations more reliable.
2. With a visualizer software, see the animation of the vibrational mode with ifreq, then freeze the animation when the vibration is in one extreme, save that structure and run again the opt/freq job.
A simplified illustration:
H-C-H ---> H-C---H ---> save and re-run opt/freq
equilibrium ..... ifreq vibration
position ........... freeze here
This example is for stretching, but the idea is for any vibration with ifreq.
Hope this help.
Best, Pablo
Thank you Pablo Mtz for your strategies.
However, in my case, things are a little more complex. The low imaginary freq. arises from the influence of two alkyl group rotations, the frequency animation reflects.
I have used method 1 but it reduced the imaginary freq. The second method does not help more.
One of the most common ways to eliminate the unwanted second imaginary frequency is to distort the molecule along the undesired vibrational mode and submit a new calculation. This method is commonly known as "Screwing".
This is a very effective method! I have used it to tackle rotational imaginary frequencies for large and fluxional molecules.
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