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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