When Minuit needs to be guided to the ``right'' minimum,
often the best way to do this is with the FIX
and RELEASE commands.
That is, suppose you have a problem with ten free parameters,
and when you minimize with respect to all at once, Minuit goes to
an unphysical solution characterized by an unphysical or unwanted
value of parameter number four.
One way to avoid this is to FIX parameter four at a ``good'' value
(not necessarily the best, since you presumably don't know that yet),
and minimize with respect to the others.
Then RELEASE 4 and minimize again.
If the problem admits a ``good'' physical solution, you will
normally find it this way.
If it doesn't work,
you may see what is wrong by the following sequence
(where xxx is the expected physical value for parameter four):
SET PARAM 4 xxx
FIX 4
MIGRAD
RELEASE 4
SCAN 4
where the SCAN command gives you a picture of FCN as a
function of parameter four alone,
the others being fixed at their current best values.
If you suspect the difficulty is due to parameter five,
then add the command
CONTOUR 4 5
to see a two-dimensional picture.