Possible reactant/input-order dependence in Geant4-DNA IRT-sync — clarification requested

Physics Processes, Models and Cross Sections
1

Geant4 version: 11.4.2
Chemistry model: IRT_syn
Platform: macOS 12.7.6, Intel x86_64

Hello Geant4-DNA developers,

We are investigating reactant-order invariance in the Geant4-DNA IRT-sync chemistry model.

As a baseline control, the unmodified official UHDR example configured with IRT_syn successfully reaches the genuine scheduler path, including:

G4ITModelProcessor::CalculateMinTimeStep
G4DNAIndependentReactionTimeStepper::CalculateStep

We then constructed a minimal scheduler-managed bimolecular test using supported Geant4-DNA molecular-track handoff paths.

The validation considers two independent ordering factors:

  1. Reactant input order:
    A then B
    versus
    B then A

  2. Reaction-registration order:
    A+B
    versus
    B+A

The four combinations were therefore:

  • AB input / AB registration
  • BA input / AB registration
  • AB input / BA registration
  • BA input / BA registration

Before the confirmatory run, we validated that true reactant-order reversal was actually occurring while preserving the same scheduler lifecycle, geometry, molecular identities, diffusion parameters, and reaction configuration.

A corrected 2×2 factorial validation was then run using predefined criteria and new independent random-seed blocks.

Multiple ordering-dependent effects were observed in the confirmatory analysis.

However, subsequent diagnostics have not been able to determine whether these observations arise from:

  • changes in RNG consumption caused by reactant ordering,
  • reactant-input implementation asymmetry,
  • reaction-registration asymmetry,
  • or a genuine ensemble/distribution-level ordering dependence inside IRT-sync.

Therefore, at this stage we are NOT asserting that this represents an implementation defect in IRT-sync.

The observation is also separate from an ordinary-IRT ordering issue that we investigated independently; we are not assuming that the same mechanism applies to IRT-sync.

Questions for Geant4-DNA / IRT-sync developers:

  1. Is reactant input order expected to alter any IRT-sync ensemble-level or distributional output?

  2. Is reaction-registration order (A+B versus B+A) expected to alter any IRT-sync distributional output?

  3. Can reversing reactant or reaction-registration order change the random-number consumption sequence while preserving the underlying ensemble distributions?

  4. Are there known reactant-identity or ordering assumptions inside
    G4DNAIndependentReactionTimeStepper
    or its associated diffusion-controlled reaction model?

  5. Is there an official or recommended regression test for verifying reactant-order and reaction-registration-order invariance in IRT-sync?

We would appreciate clarification on the intended ordering behaviour of IRT-sync and on the recommended diagnostic/regression-testing procedure.

We have a minimal NanoRT-independent reproducer, build/run instructions, preserved logs, and checksums available if they would be useful.

Thank you.