December 18th, 2015: Seasons greetings + a new ESS moderator comp released

We had hoped to have McStas 2.3 ready for release this year, but time turned out too short and we simply did not make it. Hopes are that the next release will be ready during January 2016 - very few things are missing.

In the meantime we do have a small christmas present for the ESS-oriented McStas users ready:
The ESS butterfly moderator is finally available in the download share - including support for both the 3cm and 6cm height butterfly. The release also includes a small guidance document in PDF format.

All the best, Merry X-mas and a Happy New Year from
the McStas team

September 15th, 2015: Missing manual PDF's uploaded.

A user has pointed out that no links to manual PDF's were shown on the manual page, this problem has been fixed as of today. The manuals indicate that they correspond to version 2.1, september 2014... That version and date are typos - but also an indication that no fundamental changes were made to the manual between 2.1 and 2.2a.

June 10th, 2015: McStas-2.2a supplemental ZIP archives for Mac OS X.

At least one user has experienced problems installing on OS X Yosemite from the tgz archive available here.

The problem was investigated and was caused by corruption during automatic 'double click' unarchiving of the tgz file. The solution was to unarchive using a tar command in the terminal:

tar xzf McStas-2.2a-10.10_Yosemite.tgz

If you experience this type of problem, either untar manually as shown above or try the ZIP version of the relevant archive - available here . Content wise these archives are exactly the same as the tgz version.

June 1st, 2015: Updated Ubuntu live-dvd with McStas available.

An updated iso image based on Ubuntu 14.04 and including McStas 2.2a, 2.1, 2.0 and 1.12c has been uploaded to our Ubuntu dvd archive. The iso also included the latest release of iFit and Mantid.

May 29th, 2015: Fresh built McStas 2.2a released

Dear all,

The bug reported earlier today related to the initialization of our random number generator was in fact not as severe as first anticipated:

Using a seed of 0 - which is the default in McStas and means using the clock as random number seed, instead implicitly indeed set the seed to 0 always. This meant that to generate two simulations with statistically different output, one would have had to set the seed explicitly to two different values in the two cases.

In any case, the bug has now been fixed in the newly released McStas 2.2a, which is available for all platforms at

It further seems we have an issue with our "umbrella"-metapackage (called mcstas-suite) for both Debian and RPM systems. The solution is to explicitly install the mcstas-suite-perl and/or mcstas-suite-python packages.

Best regards,

Peter Willendrup

May 29th, 2015: Random number generator bug in McStas 2.2.

Dear all,

We have unfortunately discovered a serious bug in random number generation in the newly released McStas 2.2.

A patched replacement-release will be released either today or early next week. Sorry for the inconvenience this may have caused you.

Best regards,

Peter Willendrup

May 27th, 2015: McStas 2.2 released!

Here it is: McStas 2.2 is released and ready for download via

Selected, important highligts from the release are:

Greetings from the McStas team - hope you will enjoy this new release! :-) - Full list of changes is available in the CHANGELOG

February 19th, 2015: Bugfixed-release of the ESS_moderator.comp

Jan Saroun from NPI in Czech Republic (and author of RESTRAX/SIMRES) has spotted an unfortunate error in the ESS_moderator.comp component, relating to the 2014 "pancake" case.

The bug is a missing distance-factor in a solid-angle computation and will lead to a too high brilliance emitted from the source.

Unfortunately was never spotted in our tests since these happen with an instrument where this distance is set to 1 m... (and 1 x 1 = 1... )

The updated source packge for use with McStas 1.12c and 2.x is available for download at A set of "release notes" is available at

Sorry for any inconvenience this may cause.

February 10th, 2015: Supplemental documentation for ESS moderators in McStas

A number of McStas users have expressed that the documentation for the ESS_moderator component distributed with McStas is not completely clear. To remedy this, Ken Andersen and I have (with the help of data from Troels Schönfeldt) produced a supplemental PDF document that will hopefully help.

Please note that at this point, only TDR-like and pancake moderators are described in McStas. An updated set of moderator descriptions will be released with McStas 2.2 during the spring of 2015.

Previous news items: 2014, 2013, 2012, 2011, 2010, 2009, 2008, 2007, 2006, 2005, 2004, 2003,2002, 2001, 2000, 1999, 1998.