Patrick and Franco have recently published a paper showing how to properly describe the quadrupole-allowed transitions that can be probed with X-ray Absorption Spectroscopy (XAS). Different commonly used schemes to describe transition properties were compared and their respective faults and merits were detailed. One scheme is origin dependent and violates the Thomas-Reiche-Kuhn sum rule for oscillator strengths. Another scheme is origin independent, but can result in negative oscillator strengths for dipole-allowed transitions. This work is published in the Journal of Chemical Physics and can be found here.
