Atomic data for astrophysics: Ni XII

We present new large-scale R-matrix (up to n = 4) scattering calculations for the electron collisional excitation of Cl-like Ni xii. We used the intermediate-coupling frame transformation method. We compare predicted and observed line intensities using laboratory and solar spectra, finding good agreement for all the main soft X-ray lines. With the exception of the three strongest transitions, large discrepancies with previous estimates are found, especially for the decays from the lowest 3s² 3p⁴ 3d levels. This includes the forbidden UV lines. The atomic data for the n = 4 levels are the first to be calculated. We revise previous experimental energies, and suggest several new identifications. We point out the uncertainty in the wavelength of the 3s² 3p⁵²P_1/2-3s² 3p⁴ 3d ²D_3/2 transition, which is important for density diagnostics.