ABSTRACT:   Modeling and understanding of ring current and radiation belt coupled system has been a grand challenge since the beginning of the space age. In this study we demonstrate the importance of long term simulations for the understanding of the competition between acceleration and loss processes. In particular we show that radial diffusion and scattering by whistler mode waves can reproduce the dynamics of relativistic electrons, but cannot explain the dynamics of ultra-relativistic electrons. The results of the simulations are compared to Van Allen Probes observations and the missing physical mechanisms are discussed.  We also present 4D VERB simulations that include convective transport, radial diffusion, pitch angle scattering, mixed scattering, loss to the magnetopause, adiabatic transport, cross L-pitch angle diffusion and local acceleration.