:term:`CFOUR_VIBRATION <CFOUR_VIBRATION (CFOUR)>`
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      Specifies whether (harmonic) vibrational frequencies are calculated or not. If the default NO (=0) is specified then no frequencies are calculated. For ANALYTIC, vibrational frequencies are determined from analytically computed second derivatives, and for FINDIF (=2) vibrational frequencies are calculated from a force field obtained by numerical differentiation of analytically evaluated gradients (or even single-point energies) using symmetry-adapted mass-weighted Cartesian coordinates. If vibrational frequencies are calculated, a normal mode analysis using the computed force-constant matrix is performed, rotationally projected frequencies are computed, infrared intensities are determined, and zero-point energies (ZPE) are evaluated.

      * **Type**: string
      * **Possible Values**: NO, ANALYTIC, FINDIF
      * **Default**: NO