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Compressing a FiniteMPOHamiltonian with a long-range interaction whose operators have non-zero trace lost precision geometrically with distance in v0.14.0 (#550). This was fixed by the canonical-form rewrite in #539; this test guards against regressions. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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* Update changelog for v0.14.1 Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com> * Bump version to v0.14.1 Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com> * Update CITATION.cff for v0.14.1 Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com> --------- Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>
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Closes #550.
The bug reported in #550 is already fixed on
mainby #539. This PR only adds the issue's reproducer as a regression test.Cause (v0.14.0)
In v0.14.0, the identity component of
Cwas projected out with an unnormalized identity:t = tr(1ᴴ C), without dividing bydim(P). The gauge transformation stays exact in exact arithmetic, but the identity component is never actually removed. The leftovertgrows by roughly a factor ofdim(P)per site along an interaction channel; for the MWE, the site waiting to absorb it reachednorm(C) ≈ 4e11. The result is catastrophic cancellation that gets worse with interaction distance. Traceless operators havet = 0, so they were unaffected.#539 rewrote the canonicalization as a QR of
[1 C; 0 A]with the identity as its first column, which normalizes the projection correctly.Max
|⟨H⟩ - ⟨Hc⟩|for the MWE in #550 (n ⊗ n,trunctol(rtol = 0), 5 random states):mainTest
test/algorithms/changebonds.jl: the existing "SvdCut on long-range FiniteMPOHamiltonian" testset gains the reproducer from #550:n ⊗ non sites (5, 25) of a 30-siteℂ^4chain, compressed withSvdCut(; trunc = trunctol(; rtol = 0.0)), with the expectation value checked tortol = 1e-10. The testset's other case only couples sites at most four apart, which v0.14.0 already handled correctly. On v0.14.0 the new case has a relative error of 5e-3 to 0.5; onmainit is about 1e-15.🤖 Generated with Claude Code