Systems of charged particles (such as atomic nuclei and electrons) can be described by nonrelativistic quantum mechanics with the Coloumb interaction potential. A fully relativistic description is given by quantum electrodynamics which is much more complex.

Is it possible to expand various quantities in QED as power series in 1/c i.e. around the nonrelativistic approximation? Examples of relevant quantities are:

- Ground state energy of a given set of charged particles
- Excited state energies
- Scattering cross sections of charged particles & their bound states (assuming we trace over the photons in the final state)

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