:term:`CFOUR_EVAL_HESS <CFOUR_EVAL_HESS (CFOUR)>`
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      Tells the program, in the course of a geometry optimization, to calculate the Hessian explicitly every N cycles. 0 means never calculated explicitly. **PSI4 Interface:** Geometry optimizations run through PSI (except in sandwich mode) use PSI's optimizer and so this keyword has no effect. Use :ref:`optking <apdx:optking>` keywords instead, particularly :term:`FULL_HESS_EVERY <FULL_HESS_EVERY (OPTKING)>` 

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

