:term:`CFOUR_FD_IRREPS <CFOUR_FD_IRREPS (CFOUR)>`
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      Requests that only vibrational frequencies of certain symmetry types are evaluated in a VIBRATION=FINDIF calculation. The numbers of the irreducible representations for which vibrational analysis is to be performed are separated by slashes. For example, FD_IRREP=1/3/4 means compute the frequencies of modes transforming as the first, third, and fourth irreducible representations. If a symmetry is specified for which there are no vibrational modes, the program will terminate. The labels of the irreducible representations for this keyword are not usually the same as those used in the rest of the calculation. Moreover, for some point groups, for example, those of linear molecules, the two sets of labels refer to different subgroups. There is as yet no straightforward way to determine what they will be without starting a calculation. If one runs the ``xjoda`` and then the ``xsymcor`` executables, the relevant irreducible representations will be listed. If all vibrational frequencies are desired, this keyword need not be included. Default : compute vibrational frequencies for all irreducible representations

      * **Type**: array
      * **Default**: No Default

