2 The IP-X calculation

Following the general policy of the IP, collision strengths for Ar III were computed in IP-X with a suite of programs based on the R-matrix method (Burke et al. 1971) and asymptotic techniques developed by Seaton (1985). Target wavefunctions were obtained with the atomic structure code SUPERSTRUCTURE (Eissner et al. 1974; Nussbaumer & Storey 1978; Eissner 1991). The target representation adopted included the 24 terms arising from the configurations: $3{\rm s}\sp 23{\rm p}\sp 4$,$3{\rm s}3{\rm p}\sp 5$, $3{\rm s}\sp 23{\rm p}\sp 33{\rm d}$ and $3{\rm p}\sp 6$. Configuration interaction was limited to single and double excitations within the n=3 complex. The $\lambda_{\rm nl}$ scaling parameters in the Thomas-Fermi statistical model potential used to generate the $P_{\rm nl}(r)$ radial orbitals were adjusted by minimising the weighted sum of the energies of the 24-term target model. Following the earlier five-state R-matrix calculation by Johnson & Kingston (1990) on this system, partial collision strengths were computed for $L\leq 9$.The energy range was taken to be E<3.7 Ryd, and effective collision strengths were determined for the electron temperature range $10^3\leq T\leq 10^5$ K.