Solvers and Accuracy

Rumoca ships several integration methods behind one --solver flag (CLI), solver key ([sim] in scenarios), or Solver experiment annotation.

Choosing a Solver

Rules of thumb:

• Start with auto.
• Stiff systems — fast and slow dynamics together (chemical kinetics, stiff mechanical contacts, mu-large Van der Pol) — need an implicit solver; explicit methods crawl with tiny steps.
• Models that switch rapidly (relays, hysteresis controllers) currently run most robustly with rk-like; the implicit path can stall with a step size too small error near dense event cascades. Pin it in the model: annotation(experiment(Solver = "rk-like")).

Tolerances and Output Interval

Implicit solvers control local error against relative/absolute tolerances. Sources, in priority order:

1. CLI/scenario settings (--dt, [sim] dt).
2. The model’s experiment annotation: Tolerance and Interval are used by scenario and browser runs.
3. Runtime defaults.

--dt (or [sim] dt) sets the output interval — and the fixed step for the explicit runner path. If omitted, the runtime chooses automatically.

Experiment Annotations

annotation(experiment(
  StartTime = 0.0,
  StopTime = 30.0,
  Tolerance = 1e-6,
  Interval = 0.01,
  Solver = "rk-like"
));

StopTime, StartTime, Tolerance, Interval, and Solver (also accepted: Algorithm, __Dymola_Algorithm) are parsed from the annotation. Browser/live runs and the playground honor them fully; native direct CLI runs currently take end time from --t-end (default 1.0 s) and scenario runs from [sim].

Events and the Solver

All solvers cooperate with the event system: zero crossings from when conditions and if-expressions are located, the state is re-initialized (reinit), and integration restarts cleanly at the event instant. Sampled events (sample(t0, period)) are scheduled exactly, not detected.

Try It

The Van der Pol oscillator becomes stiff as mu grows. Compare solvers by editing the annotation — try mu = 1000 with Solver = "bdf" versus Solver = "rk-like" and watch the run time in the status line:

model VanDerPolStiff
  parameter Real mu = 1000.0 "Try 1, 5, 1000";
  Real x(start = 2.0);
  Real y(start = 0.0);
equation
  der(x) = y;
  der(y) = mu * (1 - x^2) * y - x;
  annotation(experiment(StopTime = 3000.0, Solver = "bdf"));
end VanDerPolStiff;