  The parent paper to this Addendum describes the optimisation theory on which
VPFIT, a non-linear least-squares program for modelling absorption spectra, is
based. In that paper, we show that Voigt function derivatives can be calculated
analytically using Taylor series expansions and look-up tables, for the
specific case of one column density parameter for each absorption component.
However, in many situations, modelling requires more complex parameterisation,
such as summed column densities over a whole absorption complex, or common
pattern relative ion abundances. This Addendum provides those analytic
derivatives.
