:term:`CFOUR_TREAT_PERT <CFOUR_TREAT_PERT (CFOUR)>`
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      Specifies whether in a correlated NMR chemical shift calculations all perturbations are treated at once or sequentially. Available option are SIMULTANEOUS (=0) and SEQUENTIAL (=1). The latter is at least preferred for large-scale calculations, as it has less demands on the available disk space.

      * **Type**: string
      * **Possible Values**: SIMULTANEOUS, SEQUENTIAL
      * **Default**: SIMULTANEOUS