These costumes should be pretty easy to figure out. If your character is a chemist/chemical engineer/pharmaceutical scientist - opt for the mad scientist costumes or just throw on a white lab coat. 

For the robotics engineers/mechanical engineers - you can wear the lab coat or dress as one of your robot inventions and get a laugh or two.

Computer scientists and hackers can also wear a lab coat (as you might be working in a clean room), or wear a black hoodie with the Mr. Robot mask, or any semi-creepy or creepy mask (you won't need to wear it during the whole party - just enter with it).  The other mask you can wear is a Guy Fawkes, but there are no links above for that. 

With any of these super smart roles, you can always just opt for a traditional nerd with high-water pants, a button-up shirt, a bow tie, and glasses with white tape on the bridge. 

For the inventor roles, wear a lab coat, be a nerd, or give a nod to historical or even fictional inventors.