par "reset"
This command defines the relation between fit parameters (p[i])
and experimental and structural parameters, e.g. x[i]. There is
no predefined sequence or order. For each parameter p[i] that is
used in the definition, the corresponding partial derivative must
be given. The user has also to take care of proper starting
values for the parameters which can be assigned with p[i]=<expr>.
The parameter settings can be deleted using the argument "reset".
Note that all expressions within arguments ([]) are evaluated
after the command is entered, thus parameter definitions can be
made using loops. However, other variables are saves as variables
and may change during the fit. For a detailed discussion refer to
the PDFFIT users guide. Here are some examples.
Examples
par x[1]=p[1],1.0 : x of atom 1 is refined using p[1]
par o[1]=r[1]*p[5],r[1] : occupancy of atom 1 is r[1]*p[5] and
the derivative is r[1] (simple isn't it)