Installing ACE Libraries

    In order to use the libraries we provide on this site, the data needs to be installed where your code (e.g., MCNP®) can see it. In the coming weeks, tools to facilitate and automate the installation process will be available here. For now, manual installation is required.

    File/Directory Hierarchy

    Each ACE library that can be downloaded from this site has the same file/directory hierarchy when decompressed. As an example, let's look at the hierarchy for the Lib80x library.

    • Lib80x/ There will be some data directory that contains all the data files. This is required, but the directory name will be different for each library.
    • xsdir A file containing all the entries for the XSDIR file. This is required.
    • xsdir_2.0 A file containing all the ACE 2.0 entries for the XSDIR file. This may not be included.
    • docs/ A directory with documentation about the library. This exists for every production library, but probably won't exist for beta libraries.

    Install Location

    To enable your code to use the library that has been downloaded, you must put it in a location that can be seen by your code. MCNP® looks for data located in a directory pointed to by the $DATAPATH variable. This directory contains a file called xsdir_mcnp<ver> where <ver> is some version number like 6.1 or 6.2, etc. The xsdir file (as we'll call it) tells MCNP® what data is available and what files should be read to access the data. (For more information about the $DATAPATH variable and where MCNP® looks for the data, look at the MCNP® documentation.)

    Install Instructions

    Decompress the tarball downloaded from this site.

    tar -xzvf <Lib80x.tgz>

    Copy/move the data directory to location pointed to by the $DATAPATH variable

    mv Lib80x/Lib80x $DATAPATH/

    Add entries to the xsdir_mcnp<ver> file for the new data. These entries should be included after the line in the xsdir_mcnp<ver> file that is simply:


    This line will be several hundred lines down from the top of the file. See the section XSDIR File Structure below for more information about the structure of the xsdir file.

    Note: The order in which the entries appear in the xsdir file is important; the entries that appear first have precedence over those that come later. If someone specifies 92235 on their MCNP® Material Card, the first entry in the xsdir file that begins with 92235 will be used. If you want the new data to take precedence over the old data, add the entries before the existing ones. If you want your existing data to take precedence, add the new entries after the existing ones.

    XSDIR File Structure For a complete description of the xsdir file, please consult the MCNP6 manual.

    The xsdir file is broken into two main parts.

    Atomic Weight Ratios This begins with the line:


    After this line is a list of ZAs and the associated atomic weight ratio for every possible ZA that may be used in your code. An example such is given below.

    Directory. After the listing of Atomic Weight Ratios, a line containing a date (presumably when the file was last modified) followed by the line:


    After this directory line, all the entries are given. These entries tell your code how to access the available data.