In the Compton process (e.g. G4KleinNishinaCompton), the final kinetic energy of the electron is evaluted by E_k,electron = E_gamma,initial - E_gamma,final.
However, from the relativistic dynamics’ point of view, it is the total energy not the kinetic energy that conserved during interaction. Thus we should have E_total,electron = E_gamma,initial - E_gamma,final + m_electron. And then we can evaluate E_k,electron = sqrt(E_total,electron^2 - m_electron^2).
Do I miss some thing here or is Geant4 not accuracte at low energy scale (~1MeV)?
I am not sure that understand the point here: in the Compton scattering, the e- already exists (and it is an atomic electron), so this contribution is the same for the initial and the final state. So, in this case, the total kinetic energy is conserved, in addition to the total relativistic energy.