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.