The software package McStas is a tool for carrying out Monte Carlo ray-tracing simulations of neutron scattering instruments with high complexity and precision. The simulations can compute all aspects of the performance of instruments and can thus be used to optimize the use of existing equipment, design new instrumentation, and carry out virtual experiments for e.g. training, experimental planning or data analysis. McStas is based on a unique design where an automatic compilation process translates high-level textual instrument descriptions into efficient ISO-C code. This design makes it simple to set up typical simulations and also gives essentially unlimited freedom to handle more unusual cases. This report constitutes the reference manual for McStas, and, together with the manual for the McStas components, it contains documentation of most aspects of the program. It covers the various ways to compile and run simulations, a description of the meta-language used to define simulations, and some example simulations performed with the program.

This report documents McStas version 3.8.6, released September, 2026

The authors are:

Peter Kjær Willendrup <pkwi@fysik.dtu.dk>
Physics Department, Technical University of Denmark, Kongens Lyngby, Denmark

Emmanuel Farhi <emmanuel.farhi@synchrotron-soleil.fr>
Synchrotron SOLEIL, Saint-Aubin, France

Kim Lefmann <lefmann@nbi.dk>
Niels Bohr Institute, University of Copenhagen, Denmark

as well as authors who left the project:

Erik Knudsen <erkn@fysik.dtu.dk>
Physics Department, Technical University of Denmark, Kongens Lyngby, Denmark Jakob Garde <jaga@fysik.dtu.dk>
Physics Department, Technical University of Denmark, Kongens Lyngby, Denmark Jonas Stein <Jonas.Stein@uni-koeln.de>
Institute of Physics II, University of Cologne, Germany Peter Christiansen <pchristi@hep.lu.se>
Materials Research Department, Risø National Laboratory, Roskilde, Denmark
Present address: University of Lund, Lund, Sweden
Klaus Lieutenant <k.lieutenant@fz-juelich.de>
Institut Laue-Langevin, Grenoble, France
Present address: Forschungs-Zentrum Jülich, Germany
Kristian Nielsen <kristian-nielsen@mail.tele.dk>
Materials Research Department, Risø National Laboratory, Roskilde, Denmark
Presently associated with: MySQL AB, Sweden

ISBN 978–87–550–3679–6

ISSN 0106–2840

Physics Department \(\cdot \) DTU \(\cdot \) 2026