The problem description is in one file*, the input file. This file contains geometry description (cells defined with surfaces), materials, neutron source, calculation * Note that the neutron source may be defined in a separate file.

type (critical or standard), etc. This file has a special format. It contains delimiters, data entries (or cards) and comments. Each line is limited to 80 char­acters; but a single data entry may lie on more than one line if each of these lines (except the last one) ends with an & (ampersand). MCNP is case insensitive.

The general structure of the input file is:

Title line

Cell cards

Blank line delimiter

Surface cards

Blank line delimiter

Data cards

Blank line terminator

Comments can be inserted on separate lines: the line must begin with a ‘c’ followed by at least one blank. They can also be placed at the end of a data entry: after a $ (dollar sign).