electron and proton in electric field

Hello
my project is about interaction laser pulse with atomic cluster so my cluster is hydrogen cluster.so for this i used 2 separated groups of particles as electron and proton.i used proton as background because it is heavier than electron so it moves hard and just electron moves by electric field.but i can not simulate protons… please help me. how can i use both them in one input code?
Best Regards

Fatemeh