negative shape parameter for Voigt profile gives funky FWHM

[Brian]

Hello,

Currently when using Voigt profiles, fityk will return a negative shape parameter.  This seems to be identical to a positive shape parameter, with regards to the actual shape of the peak.  However, it gives a drastically too small FWHM.  You can easily test this by just creating a Voigt function, and simply flipping the sign of the shape.  The actual shape of the peak doesn't change, but the calculated FWHM does.  This is kind of problematic because even if I manually set the shape to positive, running the fit will often again result in a negative shape.  I also fit a lot of spectra, and often just miss this, and when I go to plot all my data, I get a couple of ridiculous FWHM.

Thanks,

Brian

[Dan Parshall]

That's not a bug; it's a feature.

If you want to keep your FWHM positive, use variable substitution to apply constraints.  There's a section in the manual about this, but basically it's something like:

$z=~0.1
$fwhm=$z^2

Now, no matter what value $z takes, the FWHM will be positive.

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Dan Parshall
Shorty George Productions
Boulder, CO