Why do we need vibrational frequencies?
We need the vibrational frequencies in order to compute
the vibrational zero-point energy.
When we calculate the electronic energy,
the energy is the minimum on the potential energy surface.
We need the energy of the v=0 vibrational level, which is slightly higher.
This offset is the vibrational zero-point energy.
See the figure in section
IV.C.3. Notes on Vibrational scaling factors.
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