McCode development summary, November 2026

Covers 11 Oct – 11 Nov 2026.

Three releases went out: 3.10.0 (23 Oct), 3.10.1 (30 Oct) and 3.10.2 (2 Nov). 3.10.3 is in draft. About 570 commits landed on main, and mccode-antlr went from 0.34 to 0.36.

Things that change your workflow

Results change for a fixed seed. Every ray now gets its own independent seed (default KISS generator), which removes repeated sequences across MPI ranks and GPU batches. Reference data from before 3.10.0 will not reproduce.
NeXus is now written with plain HDF5. napi/libNeXus is no longer a dependency. Files are compressed by default.
MPI event lists are split per rank. They are written as one HDF5 file per rank, joined by virtual data sets, so mccode.h5 is not self-contained. Run h5repack before moving it; the run prints how.
Install locations moved. Debian/Ubuntu packages now come from mccode-dev.github.io/debian, replacing packages.mccode.org. macOS and Windows bundles are attached to the GitHub releases.
MCPL defaults changed. MCPL_input_once is obsolete, and MCPL_input now does the MPI splitting plus repeat_count. MCPL_output defaults to weight_mode=1 (the MCPL stat:sum convention).
cif2hkl 1.5.0 or newer is required. Structure factors are now given in barns.
Test headers are renamed. %Example: becomes %Test: and %Scan: becomes %TestScan:; the old names are still accepted. mctest gains --nsigma/--pvalue acceptance, -j for parallel compiles and -n0 for compile only.

Physics fixes that change numbers

Planar monitors: about 40 of them (PSD_monitor, L_monitor, …) now restore the ray on a miss; previously such rays could be lost.
McStas samples: Phonon_simple and the Union PhononSimple_process give about 12.7× higher intensity (Squires normalisation). The Union IncoherentPhonon_process weight is fixed, and the conic mirrors (conic.h) are corrected.
McXtrace: Bending_magnet flux, Mirror_curved and Single_crystal absorption are fixed. SOLEIL beamlines are realigned, including the Si lattice constant. Source_lab now uses xraylib.
Coming in 3.10.3:
- McXtrace lenses are corrected (Lens_parab, Lens_CRL_RTM, …).
- Single_crystal_inelastic no longer depends on neutron order or MPI split.
- read_table-lib handles decreasing tables correctly.

GPU / OpenACC (3.10.2)

Newly ported to GPU:
- the 16 standalone MCViNE_* sample components;
- McStas SANSNanodiscs*, SANSCylinders, SANSEllipticCylinders and SANSPDBFast, which also no longer need GSL;
- the matching McXtrace SAXS variants (Nanodiscs, Cylinders, EllipticCylinders, PDBFast).
GPU race fixes in existing components, from a systematic component review:
- samples: Isotropic_Sqw and the SANS sphere, shell and curve samples;
- optics: gravity guides, transmission polarisers, PerfectCrystal and SupermirrorFlat.
Anyone running these on GPU should re-check older results.
CI: a compile-only OpenACC (nvhpc) leg now catches GPU build breaks on every PR.

For component and instrument authors

Name clashes: do not use I as an identifier, since it clashes with <complex.h>. Generic globals were prefixed: sign → conic_sign, square/cube → sas_square/sas_cube.
Removed or changed grammar: DETECTOR_OUT_3D is gone. An INHERIT section written in the child, even an empty one, now replaces the parent’s.
Meshes: the new geometry-mesh-lib reads STL, OFF and PLY. Union_mesh uses it now, and Monitor_nD geometry= will from 3.10.3.
Drawing: mcdis_pencolor("#rrggbb") switches pen colour inside MCDISPLAY, and many components draw filled surfaces at --trace=2.
Pull requests: the PR template must be filled in, and AGENTS.md now sets the rules for AI-assisted contributions.

New tools and infrastructure

Plotting: mcplot, mcplotdiff and mccoplot read NeXus output. mcplot-plopp and mcdisplay-plopp (Scipp/Plopp) are available as previews.
Display: mcdisplay-webgl-classic shows --trace=2 shapes by default, with Surfaces/Lines toggles and double-click to focus (the latter in 3.10.3).
Timing: from 3.10.3, wall-clock time for INITIALIZE, TRACE and FINALLY is written to mccode.sim and NeXus.
Nightly sites: the McStas and McXtrace test suites, and the ESS instruments run against main, publish results every night.
CI: a FreeBSD source-build track was added.

Websites and release cycle

New landing page at mccode.org. It links everything below. All of it is now served from GitHub Pages, built from repos in the mccode-dev organisation.
- mcstas.org and mcxtrace.org: rebuilt hourly, and pick up a new release (version, manuals, component docs) by themselves.
- mccode.org/debian: the apt repository for amd64 and arm64, including cif2hkl. It is refreshed every 3 hours and carries a new release once its builds are attached.
- Nightly test results: McStas, McXtrace and ESS instruments.
- Docs for mccode-antlr, McStasToX and McStasScript.
Still on the old DTU server: packages.mccode.org is marked inactive and points to the new apt repository. download.mcstas.org still holds the old download archive, but a 00_README.TXT at the top marks it as deprecated: from 3.10.2, use the binaries attached to the GitHub releases and the install pages on GitHub.
Release cycle is written down in docs/RELEASE-CHECKLIST.md:
- Publishing a GitHub release with tag vX.Y.Z triggers the rest: platform builds are attached after about 1 hour, then the apt repository and both websites follow.
- Install instructions are updated by an automatic PR. conda-forge follows through the feedstocks.
- Major and minor releases (4.0.0, 3.11.0) are announced on the mailing lists. Bugfix releases (3.10.3) are not, so watch the GitHub releases page for those.

McStasScript

Two releases: 0.0.96 (22 Oct) and 0.0.97 (10 Nov).

Components that use DEFINE COMPONENT ... INHERIT now show their inherited parameters.
The matplotlib instrument view supports component opacity and custom colours, and axis limits in the 2D projections.
Package imports are lazy, and IPython is an optional dependency.
A Windows problem running mcdisplay-webgl-classic from Python is fixed.