It's easier and more precise to say something like "run the quantum Monte Carlo code" than "run the quantum Monte Carlo model" or "run the quantum Monte Carlo software."

A quantum Monte Carlo code will of course include a model, but I think people don't want to call it "software" because it's so research-grade and janky. "Program" seems better, but I think that implies that it's a static thing (not in a constant state of development).

The plural "codes" is used because usually a research team has historically implemented a bunch of models into disparate codebases.