Figure 1:
The term structure of Cl V.
The horizontal dashed line indicates the cutoff of the CC expansion.
The number of terms in each electron configuration is given in brackets |

A schematic diagram of the term structure of Cl V is given
in Fig. 1, where the dashed line indicates the cutoff in the close-coupling expansion
for this ion. As the effective charge of the ions in the series decreases, the separation in energy between
states of the *n*=3 complex and the 3s^{2}*nl* Rydberg states decreases so that for P III
the close-coupling expansion needs to be extended to include terms 3s^{2}5s ^{2}S and 3s^{2}5p
^{2}P, which lie among the terms of the configuration 3s3p3d.
The cutoff of the close-coupling expansion for the three ions, P III, S IV and
Cl V,
was such that respectively 22, 21 and 21 target states were included, as shown in Tables 2, 3 and 4.

The program SUPERSTRUCTURE by
Eissner et al. (1974) as modified by
Nussbaumer & Storey (1978) was used to construct the
target wavefunctions.
The scaling parameters were optimised in two stages, first, the sum of the
energies of all configurations was minimised, and in the second stage, the
potential parameters for orbitals with principal quantum numbers *n*=4 and 5
were kept fixed whilst the others were optimised so as to minimise the energies
of all the target terms that were to be retained in the CC
expansion. This method ensured that the *n*=4 and 5 valence electron states
were not dragged into the core by the optimisation process. Whilst this
optimisation was carried out in LS coupling the contributions of the mass
and Darwin relativistic operators were included at all further stages of the
calculation.

The term systems of P III and S IV are well known (Martin et al. 1985) so that the calculated energies could be checked against experiment. The analysis is less complete for Cl V. The term structure of these ions shows some interesting configuration interaction. Terms of configurations 3p

,Martin et al. (1985, 1990) ask for further calculations of mixing coefficients. They give leading percentages alongside their term table. Therefore we give in the last Cols. of Tables 2 to 4 the theoretical target term energies () including contributions from the mass and Darwin terms, and calculated leading percentages where we adopt the presentation of Martin et al. Aashamar et al. (1984) give mixing coefficients for some terms of Al-like ions. Reistadt & Engström (1989) give leading percentages for all experimentally determined terms of S IV. Martin et al. also quote some unpublished eigenvector components from Froese-Fischer (1981).

For most terms we
agree very well with other calculated leading percentages.
However, where our calculated term energy
disagrees
significantly with experiment the associated mixing coefficients are unreliable.
Tables 2 to 4 would suggest a few changes to the configuration assignments that
might be of interest to experimentalists. Particularly
the order of terms 3s^{2}5p ^{2}Pand 3s3p(^{3}P)4s ^{2}P in P III should be interchanged.
In S IV terms ^{2}P and ^{2}D appear more strongly
mixed in our calculation than in that of
Reistadt & Engström (1989).
For Cl V we give, in Table 4, identifications according to our calculated
leading percentages that do differ from those given by
Baudinet-Robinet et al. (1982)
for some terms, we have indicated the affected terms by an *.
For a more comprehensive theoretical treatment of the term structure of Al-like ions,
Al I, Si II, S IV, Ar VI, Ca VIII, and Fe XIV,
see Mendoza et al. (1995).

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