  We present a general quantum instanton approach to calculating reaction rates
for systems with two electronic states and arbitrary values of the electronic
coupling. This new approach, which we call the non-adiabatic quantum instanton
(NAQI) approximation, reduces to Wolynes theory in the golden rule limit and to
a recently proposed projected quantum instanton (PQI) method in the adiabatic
limit. As in both of these earlier theories, the NAQI approach is based on
making a saddle point approximation to the time integral of a reactive flux
autocorrelation function, although with a generalised definition of the
projection operator onto the product states. We illustrate the accuracy of the
approach by comparison with exact rates for one dimensional scattering problems
and discuss its applicability to more complex reactions.
