Three ring current models are used to follow the evolution of the proton ring current during the 2001-04-21--25 storm: The ring current model combined with tracing particles numerically in the drift approximation by Ganushkina et al. (2005), the empirical model of proton fluxes in the inner magnetosphere developed by Milillo et al. (2003), and the kinetic ring current-atmosphere interaction model (RAM) by Liemohn et al. (2001). The paper focuses on the effects of the electric and magnetic field models and initial particle distributions on the final energy distribution. We examine a variety of large-scale magnetic field and convection electric field models as well as substorm-associated, smaller-scale, and time-varying electric fields. We find that (1) using more realistic magnetic field models leads to reduction of the ring current energy content by ~30%; (2) details of the global convection field have little influence on the overall ring current evolution; (3) smaller-scale impulsive electric field have profound effects on the ring current evolution, particularly with regard to the acceleration of the higher-energy particles; and (4) in the ring current models, the choice of the initial and boundary conditions have significant effects on the modeled ring current intensity and energy spectrum.