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.