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Method
The chi-squared minimisation is done by a combined
gradient/quadratic-fitting minimisation routine based on the
CURFIT program of Bevington. Parameter bounds are incorporated by
'pegging' parameters on bounds and excluding them from the
fitting process until the chi-squared gradient takes them back
into the allowed region. 'FROZEN' parameters are never allowed to
vary. Computing time is saved by using an approximation to the
chi-squared second derivs rather than a rigorous evaluation.
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