Element-specific relativistic corrections have been included, as in previous work. These were taken from Ref. 103 and they are 0.14 eV for C(1s), 0.28 eV for N(1s), 0.51 eV for O(1s), and 0.85 eV for F(1s), which are close to values reported elsewhere. . These corrections are added to the non-relativistic excitation energy so that the corrected excitation energy is larger than the non-relativistic result, e.g., by 0.14 eV for carbon K-edge excitation energies.