:term:`CFOUR_LINEQ_TYPE <CFOUR_LINEQ_TYPE (CFOUR)>`
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      Determines the algorithm used to solve linear equations ( :math:`\Lambda` and derivative :math:`T` and :math:`\Lambda` ). POPLE (=0) uses Pople's method of successively orthogonalized basis vectors, while DIIS (=1) uses Pulay's DIIS method. The latter offers the practical advantage of requiring much less disk space, although it is not guaranteed to converge. Moreover, POPLE has not been tested for some time and should definitely be checked! (Default : DIIS)

      * **Type**: string
      * **Possible Values**: POPLE, DIIS
      * **Default**: DIIS

