Switching classical trajectory Monte Carlo method to describe two-active-electron collisions

We propose a classical trajectory Monte Carlo method to describe two-center collisions with two active electrons. The approach is based on switching between standard four-body and three-body descriptions and it is therefore easy to implement. We demonstrate the reliability of the approach for fundamental H+H and ${\mathrm{H}}^{+}+{\mathrm{H}}^{\ensuremath{-}}$ collisions that neither four-body nor three-body classical methods describe satisfactorily.