Energy of the pair as given by the u(double) method, with application
of any PotentialTruncation that may be defined for the potential. This
does not take into account any false positioning that the Integrator may
public double collisionTime(IAtomList pair,
Computes next time of collision of two square-well atoms, assuming free-flight kinematics.
Collision may occur when cores collides, or when wells first encounter each other on
approach, or when they edge of the wells are reached as atoms diverge.