:term:`ORB_RESP_SOLVER <ORB_RESP_SOLVER (OCC)>`
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      The algorithm will be used for solving the orbital-response equations. The LINEQ option create the MO Hessian and solve the simultaneous linear equations with method choosen by the LINEQ_SOLVER option. The PCG option does not create the MO Hessian explicitly, instead it solves the simultaneous equations iteratively with the preconditioned conjugate gradient method.

      * **Type**: string
      * **Possible Values**: PCG, LINEQ
      * **Default**: PCG