This syntax highlighter for MCNP in VIM is due to Giacomo Grasso. The file on his page is inaccessible other than through the white paper, so I have taken the liberty of extracting it and putting into a file available for download. Put it inside the ~/.vim/syntax folder.
There's no easy sure-fire way of detecting an MCNP input file, but if you create a file at ~/.vim/scripts.vim with the contents
if did_filetype() " filetype already set.. finish " ..don't do these checks endif if getline(2) =~ '^C.*\<mcnp\>\c' setfiletype imcnp endif
it will load the MCNP syntax coloring if the second line of your input deck is a comment (the first letter of the line is a 'c') with the word MCNP in it somewhere.