D20 — Annealed replica exchange for parallel tempering

Decision

Parallel tempering is annealed replica exchange: the ladder (refine: 16× → 1× the annealing temperature; full: √8× → 1/√8×) follows the annealing schedule, exchanges decided at a barrier from a dedicated stream

Alternatives

Fixed ladder T0 … 1e-4·T0 (textbook PT); independent runs only

Reason

The fixed ladder accepted ~3 % of exchanges and cost +5 % vs independent runs; the annealed ladder is −4 % (refine, 23 boards) at the same move budget and identical for any thread count (doc 04 §8)