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thanks all:
I want to konw what's mean the f/i in frequency analysis?

in my opinion, this means the translation including 3 degrees of freedom

f/i stands for imaginary frequencies, which show up
a) if you are at a transition state
b) for free molecules in a box: corresponding to the translational degree of freedom of the whole molecule (this should be zero, but numerically, this is a very small number. Have a look at the eigenvector, if this is the case)