What you can share

A SolverConfig does not change after it is built. Any number of threads can read one config and build their own solvers from it.

A Solver is used by one thread at a time. If a second thread calls step or step_hess while the first call is still running, that second call raises RuntimeError and says the solver is in use. Two different solvers can run at the same time, including when the interpreter is free-threaded and sys._is_gil_enabled() is false.

The array you pass to step is updated in place. The objective receives its own copy of the coordinates, so keeping that array after step returns is safe. Do not let two threads call the same objective if the objective mutates a shared object.

Calculators

ASE calculators are not thread-safe. Each thread needs its own calculator. Sharing one calculator across threads is not supported.

rgmin guarantees three things. A SolverConfig can be shared. A Solver is entered by one thread, and a second call raises RuntimeError. The objective is handed its own copy of the coordinates.

The optimizer keeps a lock around calls on one calculator unless that calculator sets reentrant to true. The lock only stops those calls from overlapping. It does not make the calculator safe to share. import rgmin does not import ASE. The ASE classes live in rgmin.ase.

Results

The trajectory of one solver does not depend on how many Python threads or how many Rayon threads are running other solvers. The default build does not turn on the Rust parallel feature, so extra threads do not speed up a single solve.