:term:`CFOUR_FD_CALCTYPE <CFOUR_FD_CALCTYPE (CFOUR)>`
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      Specifies the algorithm used to compute the harmonic force constants in finite-difference calculations.GRADONLY (=0) evaluates the force constants and dipole moment derivatives by numerical differentiation of analytic gradients; ENERONLY (=1) evaluates the force constants by second differences of energies (dipole moment derivatives are not evaluated); while MIXED (=2) evaluates 1x1 blocks of symmetry-blocked force constants by second differences pf energies and all other elements by first differences of gradients. the GRADONLY and MIXED approaches may, of course, only be used hwen using computational methods for which analytic gradients are available.

      * **Type**: string
      * **Possible Values**: GRADONLY, ENERONLY, MIXED
      * **Default**: GRADONLY

