Finding a "free download" for MCNP6.2 (Monte Carlo N-Particle) is a common goal for students and researchers, but because it is export-controlled software, it cannot be legally downloaded like standard open-source programs.

While the software is not "free" in the sense of being public domain, there are legitimate ways to get it—sometimes with "cost recovery" fees waived for students or researchers—by following the official government process.

Two weeks later, the Dean and a representative from the Department of Energy knocked on his door. They didn't care about his thesis; they cared about the digital footprint of a federal-grade software being distributed from his IP address.

Documentation and Tools: While the code itself is restricted, you can often find official User Manuals and release notes for free on the MCNP website. How To Get The MCNP Code

Geant4: Widely used in high-energy physics and medical applications, and completely free to download.

OpenMC: A modern, community-developed code with extensive documentation and a Python API.