The legacy of MCNPX lives on in every particle simulation run with MCNP6 today. By following legal channels, you ensure reproducible science, respect for export controls, and access to the best possible code support.
. It is no longer distributed as a standalone product and has been officially superseded by Los Alamos National Laboratory (.gov) How to Obtain MCNPX
MCNP6 is the unified code. It takes the neutron/photon/electron transport of MCNP5 and combines it with the heavy-ion and high-energy proton capabilities of MCNPX. If you find an old version of MCNPX online (v2.7.0, for example), it is functionally obsolete and will not run on modern Linux compilers without serious patching.