  The theoretical description of the interplay between coherent evolution and
chemical exchange, originally developed for magnetic resonance and later
applied to other spectroscopic regimes, was derived under incorrect statistical
assumptions. Correcting these assumptions provides access to the exact form of
the exchange interaction, which we derive within the Lindblad master equation
formalism for generality. The exact form of the interaction is only different
from the traditional equation by a scalar correction factor derived from
higher-order interactions and regularly improves the radius of convergence of
the solution (hence increasing the allowable step size in calculations) by up
to an order of magnitude for no additional computational cost.
