The Hermes orbit calculations in *The Martian* were so complicated that Rich Purnell had to use some servers at the first place to complete them but when they were sent to the Hermes crew they could easily check them on their on-deck computers.

I maybe can relate to the fact that the orbital checking process can be done much faster if you already have the end results in your expense but in that case I have to ask why the orbital calculations did take so long time at the first place. A slingshot orbital maneuver seems to me as a straightforward and basic calculation which should be easily calculable even on a desktop PC.

Is there a basic misconception about the orbital checking process done by the Hermes crew or the orbit calculation done by Rich Purnell in the movie or am I completely wrong about it?