:term:`OPT_METHOD <OPT_METHOD (OCC)>`
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      The optimization algorithm. Modified Steepest-Descent (MSD) takes a Newton-Raphson (NR) step with a crude approximation to diagonal elements of the MO Hessian. The ORB_RESP option obtains the orbital rotation parameters by solving the orbital-reponse (coupled-perturbed CC) equations. Additionally, for both methods a DIIS extrapolation will be performed with the DO_DIIS = TRUE option.

      * **Type**: string
      * **Possible Values**: MSD, ORB\_RESP
      * **Default**: ORB\_RESP

