The number of parameters refined in the least-squares process.
               If possible, this number should include some contribution from
               the restrained parameters. The restrained parameters are
               distinct from the constrained parameters (where one or more
               parameters are linearly dependent on the refined value of
               another). Least-squares restraints often depend on geometry or
               energy considerations and this makes their direct contribution
               to this number, and to the goodness-of-fit calculation,
               difficult to assess.