If your molecule has N atoms, you have 3N cartesian coordinates but just 3N-6 vibrations. Most programs (I guess ORCA too) obtain the frequencies via diagonalization of the Hessian matrix in cartesian coordinates. The dimension of this matrix is 3Nx3N, but you only have 3N-6 vibrations. The other "normal modes", I mean those that have zero frequency, correspond to linear combinations of the cartesian coordinates that describe the global translations and rotations. Since the potential does not change because of these movements, you get the zero frequency. In regard to the imaginary frequency, it could be just a numerical problem, as you say. But remember you must obtain a sizable imaginary frequency if you are at the saddle point of a chemical reaction. In that case, the associated normal mode is the reaction coordinate at the TS. Last comment, if your molecule is linear you only get 5 zero-frequency modes.
If your molecule has N atoms, you have 3N cartesian coordinates but just 3N-6 vibrations. Most programs (I guess ORCA too) obtain the frequencies via diagonalization of the Hessian matrix in cartesian coordinates. The dimension of this matrix is 3Nx3N, but you only have 3N-6 vibrations. The other "normal modes", I mean those that have zero frequency, correspond to linear combinations of the cartesian coordinates that describe the global translations and rotations. Since the potential does not change because of these movements, you get the zero frequency. In regard to the imaginary frequency, it could be just a numerical problem, as you say. But remember you must obtain a sizable imaginary frequency if you are at the saddle point of a chemical reaction. In that case, the associated normal mode is the reaction coordinate at the TS. Last comment, if your molecule is linear you only get 5 zero-frequency modes.
You are right, I hadn't associated the rotational and translational modes with the zero frequencies. Nice to know that!
@@niconeuman I am glad if I could help. Thanks for the videos. I found them very useful.
love this video. thank you
Very interesting
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Thank you