Dear all, dear Steve,

(Continuing the discussion on the list, as suggested.)

Thank you, Steve, for the review of the thorn and the paper draft. The evaluation gave me a clear roadmap, and I have now completed the rework in two stages; everything is pushed to https://github.com/wylyhzh/Geodesic.

First stage (mechanical fixes):

- geodesics_integrate runs with OPTIONS: global and declares READS on the ADMBase metric; Geodesic_ParamCheck is actually scheduled; - the particle arrays are DISTRIB=CONSTANT and checkpointable; the unused CoordGauge inherit and the hard KerrSchild dependency are gone (the thorn has its own M, a parameters for the exact mode); - code fixes: the metric inversion no longer returns a garbage inverse on singular pivots; the interpolation domain guard matches the 6th-order stencil; Cactus logging was moved out of the OpenMP loop; omp.h is guarded by _OPENMP; - par/example.par now uses only public thorns and runs on a stock ET checkout; the README states the current limitations explicitly (unigrid, single refinement level, particle arrays replicated on every rank).

Second stage (tests, documentation, robustness):

- Testsuite: test/ now contains three tests, all passing under the Cactus testsuite harness: circ_orbit (full particle state compared against reference data), circ_orbit_conserved (a postproc checks that E and L_z stay constant), and circ_orbit_mpi (the same run on 2 MPI ranks, bit-identical to the single-rank result); - Documentation: doc/documentation.tex (ThornGuide format) is included; - particle loss handling is now a run-time policy (particle_loss_policy = drop / flag / respawn), with drop as the default; - the per-particle cache is allocated once and reused across steps, which removes the per-step allocation churn the review pointed out.

The remaining architectural items (correct behaviour with more than one MPI rank, interpolation through CarpetInterp/AEILocalInterp, and the time derivative of the metric from the ADMBase timelevels) are a larger rewrite whose priority I would like to discuss with the maintainers.

On the question of community interest and a published paper: the paper describing the method and the verification is included in the repository (doc/geodesic_paper.pdf). I plan to submit it to arXiv (gr-qc, cross-listed with astro-ph.HE) once it has benefited from the community's feedback, and subsequently to a journal, e.g. Classical and Quantum Gravity. I hope this addresses the "current interest" criterion in due course.

With this, I believe the thorn now meets the contribution page's requirements on documentation, examples and self-tests, within its stated scope. I would appreciate guidance on the next step, and on whether the thorn could be considered for an "incoming" listing.

Best regards,

Yuhua Li

At 2026-09-09 22:18:59, "Steven R. Brandt" sbrandt@cct.lsu.edu wrote:

Sure, we can go ahead with email. However, you should probably be sending to users@einsteintoolkit.org to address the group. See:

The current working procedure for contributions is:

Author(s) contacts the Einstein Toolkit consortium (e.g. through the users or maintainers mail lists) about software that they intend to contribute Discussions take place between the maintainers and author(s) to discuss issues above.

One question you need to address at some ponit:

To focus development and support, optimize resources, and provide best practises, the Einstein Toolkit components should be of current interest to the community. This is phrased ambiguously, but usually means there's a paper published in a reputable journal somewhere.

--Steve

On 9/9/2026 8:37 AM, Yuhua Li wrote:

Dear Steve,

Thank you for taking the time to look at the thorn and the paper, and for forwarding the evaluation. I have gone through it carefully.

I agree with the two problems you singled out — the missing READS/WRITES clauses and the MPI layout — and most of the other findings are also correct. The evaluation gave me a clear roadmap, and I have now completed the first, mechanical stage of the rework:

- schedule.ccl: geodesics_integrate now runs with OPTIONS: global and declares READS on the ADMBase metric; Geodesic_ParamCheck is actually scheduled at CCTK_PARAMCHECK; - interface.ccl: the particle arrays are now DISTRIB=CONSTANT and checkpointable; the unused CoordGauge inherit is gone; - param.ccl: the hard KerrSchild dependency was removed — the thorn has its own M and a parameters for the exact mode; - storage now follows particle_n_total instead of particle_n; - code fixes: the 4x4 metric inversion no longer returns a garbage inverse on singular pivots; the interpolation domain guard now matches the 6th-order stencil; Cactus logging was moved out of the OpenMP loop; omp.h is guarded by _OPENMP; the parameter prose and the copyright headers are fixed; - par/example.par now uses only public thorns and runs on a stock ET checkout; the README states the current limitations explicitly (single refinement level, unigrid, particle arrays replicated on every rank).

All of this is pushed to the repository, and the thorn rebuilds and runs cleanly.

The remaining, architectural items — correct behaviour under MPI with more than one rank, interpolation through CarpetInterp/AEILocalInterp, and the time derivative of the metric from the ADMBase timelevels — are a larger rewrite. I would like to discuss their priority with the maintainers before starting, and I will state the current domain clearly in the paper as well: unigrid, a single MPI process, stationary or slowly evolving metrics.

For the ET contribution requirements I still plan to add the doc/documentation.tex ThornGuide documentation and a test/ case (conservation of E and Lz on a circular Kerr orbit) — a pointer to a good example test to follow would be appreciated.

About the weekly meetings: I would honestly prefer not to join, because my spoken English — especially listening — is not good enough for a live discussion, and I cannot change that in the short term. I hope we can communicate in writing instead, here on the mailing list or by email; if a short written summary for one of the meetings would help, I am happy to provide one.

Thank you again for the constructive feedback.

Best regards,

Yuhua Li

At 2026-09-09 07:07:21, "Steven R. Brandt" sbrandt@cct.lsu.edu wrote:

Yuhua,

I had Grok take a look at your thorn and paper. You will find the feedback attached (eval.md). At least some of the things it identifies are valid problems, namely the lack of READS/WRITES clauses in the schedule and MPI support.

You may want to consider joining one of the weekly ET meetings to make your case (I realize that the time will probably be challenging for you).

--Steve

On 9/8/2026 11:00 AM, Yuhua Li wrote:

Dear Steven,

Thank you for the quick and encouraging reply, and for the pointer to the contribution process.

To answer your question directly: my goal is to have Geodesic included in the Einstein Toolkit, and I am willing to follow the process described at https://einsteintoolkit.org/contribute.html step by step. In the meantime, I would be glad if the thorn could be added to the "Thorns we know of" list.

Where the thorn stands with respect to the requirements:

- Documentation: the repository has a README with purpose, build and usage instructions and a parameter table; the algorithms and the verification are described in the paper draft (doc/geodesic_paper.pdf). - Examples: par/example.par is a complete, runnable example, and the par/ directory contains the parameter files that reproduce every result reported in the paper. I am happy to add an automated test in the ET test-suite format — a pointer to a good example test to follow would be appreciated. - Licence: GPLv2+, included as COPYING. - Hosting: the code is on GitHub (https://github.com/wylyhzh/Geodesic). I am open to hosting it at einsteintoolkit.org if that is preferred.

Please let me know what the next step should be — for example, whether the thorn can be placed into an "incoming" repository, and what the review will look at first. I will respond to any requested changes in a timely manner.

Best regards,

Yuhua Li

At 2026-09-08 22:41:39, "Steven R. Brandt via Users" users@einsteintoolkit.org wrote:

Dear Yuhua,

Thank you for writing and for putting the code and the paper draft in a public repository under GPLv2+. Independent contributions of this kind are welcome, and a test-particle geodesic tracker is something we don't have: the ET currently ships fluid tracers (particle_tracerET) and puncture tracking (PunctureTracker), but not a timelike geodesic integrator on the 3+1 metric.

Were you hoping to have it included in the ET, or were you just looking to share the code with the community? If so, we can help you with the process. Please take a look at https://einsteintoolkit.org/contribute.html if you haven't already.

If you would like to do something less formal, we can share your link on our list of "thorns we know of": https://docs.einsteintoolkit.org/et-docs/Thorns_we_know_of

Best regards,

Steven R. Brandt

On 9/8/2026 12:28 AM, Yuhua Li via Users wrote:

Dear Einstein Toolkit community,

I am a retired researcher (background in mathematics and computer science) working independently from China. Over the past ten years I have been building a test-particle geodesic tracker for the Einstein Toolkit, and I would like to share it with the community.

The Geodesic thorn integrates geodesics for large numbers of test particles in Kerr–Schild Cartesian coordinates. It supports (i) the exact Kerr–Schild metric, independent of the grid (verified symbolically to be the vacuum solution to machine precision), and (ii) 27-point local Lagrange interpolation of the grid metric, so it also works for arbitrary spacetimes carried on the grid (e.g., GRHydro evolutions). The per-particle integration is fully independent and parallelized with OpenMP (validated with 10^4 particles).

The code (GPLv2+), the configuration files used in the validation runs, and a paper draft describing the method and verification are all available in this public repository: https://github.com/wylyhzh/Geodesic

I plan to submit the paper to arXiv (gr-qc, cross-list astro-ph.HE) once it has benefited from the community's feedback.

I would appreciate: (1) feedback on whether the thorn meets ET standards, and what would be needed to have it listed with the ET thorns; (2) any comments on the paper. I am most comfortable discussing in writing, so I would prefer to continue the conversation here on the mailing list or by email, whichever is more convenient.

I am happy to provide any further information.

Best regards,

Yuhua Li

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