McStas logo McStas - A neutron ray-trace simulation package ILL; PSI; ESS Niels Bohr Institute DTU Physics NEXMAP


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McStas Frequently Asked Questions

McStas FAQ - Frequently Asked Questions with answers


Where can I get information about McStas?

A good source of information is the McStas Web page, at

Where can I ask questions about McStas?

Questions about McStas should be sent to the neutron/Monte Carlo mailing list. See for details.

Are there any bugs in McStas?

A list of known bugs can be found at Also have a look at out bug tracking system.

Installation - configuration

The installation of McStas seems rather complex. How should I proceed ?

Have a look at the Install page. We provide installers for most systems. A manual installation can be done using the procedure indicated in our SVN page.
If you really can not manage, the McStas LiveDVD provides a fully functional system for all Intel/AMD based systems, without touching your local machine, but with full computing efficiency.

The installation of McStas cannot find the right Perl interpreter. What can I do?

See the installation instructions and try out the Python-chaco, Python-matplotlib, Gnuplot or Matlab/iFit plotters.

Using McStas

How do I make the graphical user interface use another C compiler?

It is possible to define the MCSTAS_CC and MCSTAS_CFLAGS environement variables to override the default configuration, or to simply set it in the mcgui Preferences.
export MCSTAS_CC=/usr/opt/ansic/cc

I can not write bigger files than 2Gb with McStas, is there a workaround?

First, please consider if you really need such big files. If you do, McStas can perfectly well write bigger files than 2Gb if you make sure that your MCSTAS_CFLAGS contains proper c-compiler defines to allow this. On x86 Linux with gcc, relevant values are
You may also prefer the NeXus format (HDF5 based) that McStas can generate. This format can support very large data files. Look at the Install page

I think McStas gives the wrong number of neutrons in the detector in my simulation. What is wrong?

The number of neutrons histories that reach the detector in a McStas simulation is usually not very meaningful. McStas uses weighted neutrons extensively (see the manual for details) and different neutrons may carry widely different weights. Always use the intensity in detectors when analyzing simulation data.

Can McStas use clusters and Grids ?

Yes, install MPICH or OpenMPI, and select MPI or Grid when running the simulation from the GUI.

Can McStas import files from other neutron MC packages ?

Yes, McStas can read files generated by Vitess, MCNP/MCNPX, and Tripoli. Other importers are easy to write. Just ask us.

What type of data can McStas generate ?

McStas usually generates histograms (tallies) as formatted (easy to read) text files. Such text files may be read by the McStas internal plotter using PGPLOT, Python/matplotlib, Python/chaco, iFit/Matlab, and Gnuplot.
Event files may be generated for MCNP/MCNPX, Vitess and Tripoli. McStas has also its own event file format, which can be used to split a simulation into pieces (see the User Manual with the Virtual_input/output component).

Is there a way to interact with a running simulation ?

Yes, it is possible to make the simulation report and save results during the simulation. The on-line plotting tool mcdaemon may then plot the results as they come, in the course of the computation (see the Progress_bar component). On Unix/Linux/MacOSX systems (excet when running with MPI and grids), one may additionally send USR1 and USR2 signals to the simulation (refer to the User Manual).

Last modified: Fri Oct 18 16:25:25 CEST 2013

Last Modified: Friday, 18-Oct-2013 16:42:02 CEST
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