The computed absorption onset of CaCuP is also shown in Figure , considering first the static lattice independent particle absorption (IPA). This absorption onset corresponds to the minimum vertical excitation at the optical band gap located at the Γ point, with an energy value of 2.17 eV. Even though the direct transition at the Γ point is parity-allowed, the absorptivity is rather low. We propose that the second, more intense peak in the static spectrum at 2.71 eV corresponds to a transition at the Γ point from the VBM to CBM+1, with the increased intensity arising from the better overlap of the respective orbitals. Contributions from both of these transitions reasonably yield the experimentally derived direct band gap of around 2.5 eV. Convergence tests with respect to the k-point grid suggest that even though the position of the absorption onset is very well converged with a coarse k-point grid, the spectral line shape changes significantly and the extremely bright transition at 2.71 eV becomes much less prominent with finer k-point sampling (suggesting the very large increase in absorption reported in Ref. 24 may be an exaggerated effect of under-convergence).