:term:`CFOUR_ZFIELD <CFOUR_ZFIELD (CFOUR)>`
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      Specifies the Z-component of an external electric field. The value must be specified as an integer and the field used by the program will be the value of the keyword :math:`\times 10^{-6}`. This allows field strengths :math:`|\varepsilon| > 10^{-6}` to be used.

      * **Type**: integer
      * **Default**: 0