Table 1: Summary of the extended model atom. The iron-group ions (Fe) are described by a relatively small number of super-levels, each representing a large number of true atomic energy levels (sub-levels). In the present work we also apply a smaller model atom, where the iron-group ions are restricted to Fe II-Fe X with Fe X represented by one auxiliary level.

Ion	Levels		Ion	Super-levels	Sub-levels
He I	17		Fe II	1	1
He II	16		Fe III	8	14 188
He III	1		Fe IV	18	30 122
C I	2		Fe V	19	19 804
C II	32		Fe VI	18	15 155
C III	40		Fe VII	16	11 867
C IV	54		Fe VIII	10	8669
C V	1		Fe IX	11	12 366
O II	3		Fe X	9	761
O III	33		Fe XI	9	1143
O IV	25		Fe XII	9	894
O V	36		Fe XIII	9	797
O VI	15		Fe XIV	9	589
O VII	1		Fe XV	7	253
Si III	10		Fe XVI	6	31
Si IV	7		Fe XVII	1	1
Si V	1