:term:`COLLAPSE_SIZE <COLLAPSE_SIZE (DETCI)>`
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      Gives the number of vectors to retain when the Davidson subspace is collapsed (see :term:`MAX_NUM_VECS <MAX_NUM_VECS (DETCI)>` . If greater than one, the collapsed subspace retains the best estimate of the CI vector for the previous n iterations. Defaults to 1.

      * **Type**: integer
      * **Default**: 1