Caveats: guess

Initial model parameter values are no longer automatically guessed in Sherpa; the guess command is available for this purpose, but only for simple (i.e. single-component) models. It makes an initial guess at parameter values to ensure convergence, but users ought to check that the initial range of values (soft limits) is sensible for their data.

The guess command is used to set sensible initial values for thawed model parameters before a fit, however it has been found to also change parameter values which have been frozen. The frozen value gets changed to a value within the min/max range set for that parameter. This will be fixed in a future release of Sherpa.

Workaround:

To work around this bug, simply ensure that the guess command is run on a model or model component before freezing parameters to desired values.