Table 1: Fine structure levels and the relative energies of the target (core) ion Fe XXI in wavefunction expansion of Fe XX. The atomic structure calculations for the wavefunction employs spectroscopic set of configurations: 2s22p2, 2s2p3, 2p4, 2s22p3s, 2s22p3p, 2s22p3d, 2s2p23s, 2s2p23p, 2s2p23d, 2p33s, and correlation configurations: 2p33p, 2p33d, 2p34s, 2p34p, 2p34d, 2s22p4s, 2s22p4p, 2s22p4d, 2s2p24s, 2s2p24p, 2s2p24d, and the Thomas-Fermi scaling parameters for the orbitals: $\lambda _{nl} = 1.42$(1s), 1.25(2s), 1.15(2p), 1.25(3s), 1.15(3p), 1.17(3d), -1.20(4s), -1.20(4p), -1.2(4d).
  Term Jt Et (Ry)

1		 2s22p2(3P) 0 0.0
2 2s22p2(3P) 1 0.67297
3 2s22p2(3P) 2 1.0694
4 2s22p2(1D) 2 2.2286
5 2s22p2(1S) 0 3.3890
6 2s2p3(5S$^{\rm o}$) 1 4.4374
7 2s2p3(3D$^{\rm o}$) 1 7.0785
8 2s2p3(3D$^{\rm o}$) 2 7.0837
9 2s2p3(3D$^{\rm o}$) 3 7.3260
10 2s2p3(3P$^{\rm o}$) 0 8.3507
11 2s2p3(3P$^{\rm o}$) 1 8.4281
12 2s2p3(3P$^{\rm o}$) 2 8.5871
13 2s2p3(3S$^{\rm o}$) 1 9.9838
14 2s2p3(1D$^{\rm o}$) 2 10.268
15 2s2p3(1P$^{\rm o}$) 1 11.491
16 2s2p4(3P) 2 15.002
17 2s2p4(3P) 1 15.861
18 2s2p4(3P) 0 15.817
19 2s2p4(1D) 2 16.560
20 2s2p4(1S) 0 18.665


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