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Lyα MCRT

How does THOR stack up against established Lyα radiative-transfer codes on accuracy and speed?

Lyα MCRT benchmark — per-code throughput, THOR on GPU versus the CPU codes Lyα MCRT benchmark — per-code throughput, THOR on GPU versus the CPU codes

Run on a single consumer desktop — AMD Ryzen 9 5950X and an NVIDIA RTX 3090 — at N = 10 000 photons, all -O3: THOR with clang (via AdaptiveCpp), the other codes with gcc/gfortran. The GPU bar is THOR's FP32 build at N = 10⁶ (enough to fill the card); the CPU bars are FP64.2

Accuracy

The test is the Neufeld static sphere — a point source at the centre of a static, uniform, dust-free sphere of hydrogen — run against RASCAS, COLT and Iltis. Its emergent Lyα profile has a closed-form solution1, so each code's accuracy is just its R² against it:

\[ J(x) \;\propto\; \frac{x^2}{1 + \cosh\!\big[\sqrt{2\pi^3/27}\,|x^3|/(a\,\tau_0)\big]} \]

Canonical run: τ₀ = 10⁶, T = 2×10⁴ K, xcrit = 0 (no acceleration).

Emergent Lyα spectra of THOR (CPU and GPU), COLT, RASCAS and Iltis vs the analytic solution at τ₀ = 10⁶ Emergent Lyα spectra of THOR (CPU and GPU), COLT, RASCAS and Iltis vs the analytic solution at τ₀ = 10⁶

All four codes land on the analytic curve (R² ≈ 0.91–0.96) and agree with each other to ~1.5 %, each doing the ~10⁶ scatterings per photon expected without acceleration.

Iltis

The public cbehren/Iltis is broken for this comparison (spiky spectrum, R² ≈ 0.6 at τ₀ ≥ 10⁵), so it is not in the default run. Add it with a maintained (private) voroILTIS build — run_all.sh forwards ILTIS_URL/ILTIS_REF to the container build: 

ILTIS_URL=<voroILTIS-repo> ILTIS_REF=<sha> \
  run_all.sh --build --codes "thor rascas colt iltis" --tau10 6 --out ./out

Run it

apptainer/benchmarks/neufeld/common/run_all.sh --build --tau10 6 --out ./out

TAU10 sets the optical depth; NPHOTONS, RASCAS_NRANKS and --march are the other knobs.

References

  • Neufeld (1990), ApJ 350, 216 — ADS
  • Dijkstra, Haiman & Spaans (2006), ApJ 649, 14 — ADS
  • Byrohl & Nelson (2025), THOR code paper — see Papers

  1. Dijkstra, Haiman & Spaans (2006), after Neufeld (1990). ↩

  2. Run 2026-06-05. Code versions — THOR 7449d06a, COLT 88429d9, RASCAS 3b18dd0, voroILTIS 9ae77da (plus local Neufeld-setup modifications not captured by the hash). The public-Iltis container recipe pins 3efd87c but is not used for these figures. ↩