simulator's work

So essentially the simulator only does time stepping? You don’t make use of it’s force routines, etc…?

Simulators and models are separate entities. The “simulator” does the calculation and the “model” is a routine that given positions, species, etc. returns the energy, forces, virial, etc. The simulators can be time-stepping as you say (i.e. perform molecular dynamics), but it can also be molecular statics (energy minimization), Monte Carlo, lattice dyanmics, etc.