simulating atom is hard because the interaction between protons and neutrons are described by not only EM force but also Strong force, and in particular, Strong

simulating atom is hard because the interaction between protons and neutrons are described by not only EM force but also Strong force, and in particular, Strong force at this low energy (as oppose to what happens at the LHC) is hard, partly because https://en.wikipedia.org/wiki/Asymptotic_freedom https://en.wikipedia.org/wiki/Asymptotic_freedom , basically our usual physics tricks of "perturbation approximation" doesn't work because the Strong force diverge at lower energy