The use of a pseudo-state expansion within the standard low-energy R-matrix framework to facilitate the study of electron scattering by complex atoms and ions at both low and intermediate energies is discussed. Electron scattering from atomic hydrogen is considered as an example, and results for elastic scattering phase shifts and excitation cross sections are found to be in excellent agreement with recent IERM results in these energy regions. The advantage of this procedure is that existing computer codes, which have been developed over many years, can be directly extended to study electron scattering from a general N-electron target atom or ion.

Context. Absorption or emission lines of Fe ii are observed in many astrophysical spectra and accurate atomic data are required to interpret these lines. The calculation of electron-impact excitation rates for transitions among even the lowest lying levels of Fe ii is a formidable task for theoreticians.Aims. In this paper, we present collision strengths and effective collision strengths for electron-impact excitation of Fe ii for low-lying forbidden transitions among the lowest 16 fine-structure levels arising from the four LS states 3d 6 4s 6 D e , 3d 7 4 F e , 3d 6 4s 4 D e , and 3d 7 4 P e . The effective collision strengths are calculated for a wide range of electron temperatures of astrophysical importance from 30-100 000 K. Methods. The parallel suite of Breit-Pauli codes are utilised to compute the collision cross sections for electron-impact excitation of Fe ii and relativistic terms are included explicitly in both the target and the scattering approximation. 100 LS or 262-jj levels formed from the basis configurations 3d 6 4s, 3d 7 , and 3d 6 4p were included in the wavefunction representation of the target, including all doublet, quartet, and sextet terms. Collision strengths for a total of 34 191 individual transitions were computed. Results. A detailed comparison is made with previous theoretical works and significant differences were found to occur in the effective collision strengths, particularly at low temperatures.

In this paper, we report on our "Iridis-Pi" cluster, which consists of 64 Raspberry Pi Model B nodes each equipped with a 700 MHz ARM processor, 256 MiB of RAM and a 16 GiB SD card for local storage. The cluster has a number of advantages which are not shared with conventional data-centre based cluster, including its low total power consumption, easy portability due to its small size and weight, affordability, and passive, ambient cooling. We propose that these attributes make Iridis-Pi ideally suited to educational applications, where it provides a low-cost starting point to inspire and enable students to understand and apply highperformance computing and data handling to tackle complex engineering and scientific challenges. We present the results of benchmarking both the computational power and network performance of the "Iridis-Pi." We also argue that such systems should be considered in some additional specialist application areas where these unique attributes may prove advantageous. We believe that the choice of an ARM CPU foreshadows a trend towards the increasing adoption of low-power, non-PC-compatible architectures in high performance clusters.

Effective collision strengths for electron-impact excitation of Fe II are presented for all sextet-to-quartet transitions among the 38 LS states formed from the basis configurations 3d64s, 3d7 and 3d64p. A total of 112 individual transitions are considered at electron temperatures in the range 30–100 000 K, encompassing values of importance for applications in astrophysics as well as laboratory plasmas. A limited comparison is made with earlier theoretical work and large differences are found to occur at the temperatures considered. In particular, it is found that the inclusion or omission of some (N + 1)-bound configurations in the Hamiltonian matrices describing the collision process can have a huge effect on the resulting effective collision strengths, by up to a factor of four in some cases.

The reduced IERM approach is applied to electron scattering from hydrogen atoms below the n=2 and n=3 thresholds to obtain accurate widths and positions for the resonant states of H- in this energy region, together with integrated cross sections for n=1 to n=2 transitions. Comparison with standard R-matrix results, where only bound states have been included in the close coupling expansion, indicates the importance of including the continuum channels even at these low energies. The results are in good agreement with coupled pseudostate and recent J-matrix calculations.

We have applied the R-matrix with pseudo-states method (RMPS) to study electron - helium scattering at intermediate energies. Differential cross sections for elastic and inelastic collisions and electron-impact coherence parameters for excitation of the transition are presented for incident energies of 30, 50 and 80 eV. Excellent agreement with experiment and with the predictions of the convergent close-coupling calculation by Fursa and Bray is obtained. Since correlation effects and target continuum states are both accurately represented in a completely general way by this method, the associated program package can now be used to obtain reliable results at intermediate energies for arbitrary complex atomic and ionic targets of importance in applications.

Partial wave collision strengths are presented for low-energy electronimpact transitions in Fe II between the 3d 6 4s a 6 D e ground state and the 3d 7 a 4 F e , 3d 6 4s a 4 D e and 3d 7 a 4 P e low-lying excited states. The collision strengths are calculated in LS coupling using a new Fortran 95 R-matrix program including all terms of the 3d 6 4s, 3d 7 , 3d 6 4p, 3d 5 4s 2 and 3d 5 4s4p configurations in the close coupling expansion of the collision wavefunction. Special emphasis is given to the inclusion of configuration interaction (CI) effects both in the target and in the collision wavefunctions. In both cases series of calculations are carried out where additional CI terms are included systematically. It is found that in order to obtain close to converged low-energy partial wave collision strengths two-electron excitations from the 3p shell to the 3d shell as well as pseudo s and d orbitals must be included in the CI expansions. Also resonance effects in low-energy partial wave collision strengths are found to depend sensitively on the representation of the d orbitals in the CI expansion of the collision wavefunction. Finally, CI models for both the target and collision wavefunctions are defined which can be used in proposed calculations to obtain accurate total collision strengths and effective collision strengths for transitions between LS-coupled terms and between fine-structure levels of Fe II.

The R matrix with pseudostates method has been used to study e Ϫ -He collisions at low and intermediate energies up to 80 eV. Since target correlation effects and target continuum states are both accurately represented by this method, the associated program package can now be used to obtain reliable results at intermediate energies for more complex atomic and ionic targets, which are urgently required in many applications.