feat: BUG integrator#468
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Renormalization after each step is now controlled by an explicit `normalize` kwarg on `timestep`/`time_evolve` (default `false`) instead of being tied to `imaginary_evolution`. By default the norm is preserved, so it retains useful information (accumulated truncation error in real time, decaying weight in imaginary time); pass `normalize = true` to renormalize, e.g. for ground-state or thermal-state search. Threaded through the driver and both TDVP/TDVP2 integrators; test norm/imaginary-time expectations updated accordingly. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Add the `BUG` time-evolution algorithm: a symmetric second-order Basis-Update & Galerkin integrator for finite MPS. Unlike `TDVP` it has no backward-in-time substep (stable for imaginary-time evolution); passing a truncating `trscheme` enables rank-adaptivity, growing and shrinking the bond dimension to track the entanglement. Wires up the export, include, docs (`@docs` entry, changelog, Ceruti & Lubich 2022 reference). Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Add tests for the sequential `BUG` integrator: energy conservation, eigenstate phase, agreement with TDVP, second-order convergence vs dense propagation, imaginary-time cooling, LazySum/time-dependent operators, rank-adaptivity, and symmetric tensors (U(1)/Z2/SU(2), incl. rank-adaptive-symmetric). Plus unit tests for the `_bug_augment_*` basis-augmentation helpers. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
- Remove unused `tolgauge`/`gaugemaxiter` fields from the `BUG` struct and constructor: they are read only for `InfiniteMPS` gauge reconstruction in TDVP, and BUG is finite-only, so they were silently ignored. - Clarify the `BUG` docstring (single-site, symmetric second-order, finite MPS) and correct the `finalize` signature docs (`finalize(t, …)`, not `iter`) in BUG, TDVP and TDVP2. - Document the `normalize` behavior change under changelog "Changed": it is now an explicit kwarg (default false) decoupled from `imaginary_evolution`; imaginary-time evolution no longer renormalizes unless `normalize = true`. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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July 21, 2026 01:10
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This PR adds the BUG integrator as an alternative to TDVP. It's a consolidation of part of #454.
The main purpose is to have a stable integrator to be able to handle (large) imaginary or complex time steps in a way that is more stable than standard TDVP since it does not involve a backwards integration.
@Jutho I would be very happy to hear your opinions on this, I'm still not completely sure I've done everything correctly and many parts still confuse me, so a second set of eyes would be greatly appreciated. 🙃