Tips & Suggestions

SOFT gives a ‘Step-size underflow in Runge-Kutta integrator’ error.

This error is usually due to one of two reasons: (i) something is wrong with the magnetic field you are running with, or (ii) the integrator tolerance @Equation tolerance is too strict.

The former reason is possible only when using a numerical magnetic field and is indicative of a problem in one or several of the magnetic field components. You should check to make sure that the magnetic field actually looks fine before proceeding.

The latter reason occurs if @Equation tolerance is set to a too small value. Very few applications require an accuracy in the computed quantities to machine epsilon, and for both performance and stability reasons, @Equation tolerance should therefore be set to a value significantly larger than machine epsilon. On most systems, machine epsilon is around \(2\cdot 10^{-16}\).