Topological charge unfreezing with AMReX

A new approach to the problem of topological freezing in gauge theories is introduced in which a physical volume preserving coarsening of the lattice induces sufficient energy variation in the Hamiltonian to overcome large topological barriers. Though the process is not reversible, the physical volume preserving aspect minimises the time spent rethermalisating the lattice after coarsening periods, which we then treat as a new ensemble disjoint from previous runs. We have tested this technique on the pure gauge 2D Schwinger model and find that topological sampling rates are improved. We also demonstrate that autocorrelation times for extensive observables are restored to their pre-coarsening values after coarsening periods over a acceptably short simulation time.