Not much difference here, Calculation (or, more generally, Computation) is the manipulation of abstract symbols according to pure rules that may or may not represent concrete entities, e.g. the simplification of polynomials according to the rule of adding like powers.

Simulation is when we manipulate things (concrete or abstract) according to the rules that govern other concrete things, e.g. pushing around balls in circles to (highly inaccurately) represent the orbit of planets around a star.

Not all calculation is simulation, and not all simulation is calculation, but there exists an intersection of both.

The key trick you can do with that last category is that when the physical system you're simulating is controllable enough, you can use the correspondence in the other direction: Use the concrete things to simulate the abstract things. It's simulation, because you're manipulating concrete entities according to the rules that govern other entities (who happen to be abstract),but what you're doing also amounts to doing a calculation with those abstract entities.