Summary

test_default_fallback (added in #5491) compared neighbor_list=None against an explicit DefaultNeighborList() with exact equality (assert_array_equal). The two are the identical builder — call_common does builder = nl if nl is not None else DefaultNeighborList() — so on CPU the two forward passes are bit-identical. But the test runs two independent forward passes, and on CUDA the dpa3 GNN message-passing scatter (atomic adds) is not bit-reproducible run-to-run. The virial differed by ~1 ULP (abs 3.46e-18, rel 5.64e-16), failing the exact comparison intermittently:

FAILED test_default_fallback[dpa3] - AssertionError: Arrays are not equal
dpa3 virial — Mismatched elements: 1 / 9 (11.1%)
Max absolute difference: 3.47e-18 ; Max relative difference: 5.64e-16

Fix

Switch the comparison to assert_allclose(rtol=1e-10, atol=1e-12) — far above the fp noise floor (~1e-16 rel) but orders of magnitude tighter than any real dispatch divergence (e.g. accidentally using a different builder) would produce. Verified on CPU that None and DefaultNeighborList() are bit-identical (max|Δ| = 0), confirming the residual is CUDA atomic nondeterminism, not a dispatch bug.

Known limitations

• Validated on CPU (8/8 test_default_fallback pass). The failing CUDA path could not be re-validated locally (no GPU available this session); the CI CUDA job is the confirmation. The tolerance is ~6 orders of magnitude above the observed noise, so it is robust.
• Relaxes the previously-exact comparison; this is the intended change. The sibling test_pt_expt_equivalence tests already use a 1e-9 tolerance.

Summary by CodeRabbit

Release Notes

• Tests
  • Enhanced test robustness by implementing floating-point tolerance comparisons instead of strict equality checks, ensuring reliable testing across different hardware configurations.