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Frida contains a quite extensive set of routines to fit data.
For fitting of the data a Gauss-Newton algorithm is employed at the moment.

Fitting of the data makes use of the E04FCF routine of the NAG FORTRAN library.

Frida allows to adjust a number of parameters such as the tolerance of the fit
etc.. The following page lists a description of the different commands available
in the curve fitting menu c.

  • cc creates a curve. The curves are selected by number. The available curves are
    specified in the file i66.f. Some of the curves that make use of externally provided
    numerical tables have subroutines defined in the file i67.f.
  • cf executes the fit of the curve created by cc to the data. Upon execution
    this command asks for the file number of the data and possibly a file to convolve this
    analytical function with, as well as which spectra of the file should be fit.
  • cp allows to change the parameters of the curve created with cc at any time.
  • cpa parameters (alt)
  • ca specifies auxiliary parameters, which are the fit range, and wether
    the fit shall be weighted with stepwidth in x, error bars associated to the
    data points, or logarithmically. The latter takes especially into account if
    data is varying over several orders of magnitude as it is typically the case for
    scattering laws measured by means of quasielastic neutron scattering. Moreover, a
    flag can be set to specify that the analytical curve shall be convolved with a
    another curve (e. g. resolution function).
  • cnn new number
  • cs allows to change standard parameters for the fit routine. The parameters
    are …
  • ci allows to extract the fit parameters from the fit curve saving them into
    an integral file. Imagine the following: The scattering law S(q,w) has been
    measured for several different q-values. S(q,w) is fit to a model function
    e. g. a Lorentzian. The linewidth HWHM is one of the fit parameters that varies
    with q. ci extracts this parameter with q as the x-grid and HWHM as
    the y-grid.
  • cg creates a file that contains the curve on a grid which can be specified to be
    the same as the data-file, linear, logarithmical, semi-lograthmic or user-specified.
    This is e. g. useful if one wants to subtract the fit-curve from the data file or the direct display of the function yields oscillations.