  We present a simple interpolation formula for the rate of an electron
transfer reaction as a function of the electronic coupling strength. The
formula only requires the calculation of Fermi Golden Rule and Born-Oppenheimer
rates and so can be combined with any methods that are able to calculate these
rates. We first demonstrate the accuracy of the formula by applying it to a one
dimensional scattering problem for which the exact quantum mechanical, Fermi
Golden Rule, and Born-Oppenheimer rates are readily calculated. We then
describe how the formula can be combined with the Wolynes theory approximation
to the Golden Rule rate, and the ring polymer molecular dynamics (RPMD)
approximation to the Born-Oppenheimer rate, and used to capture the effects of
nuclear tunnelling, zero point energy, and solvent friction on condensed phase
electron transfer reactions. Comparison with exact hierarchical equations of
motion (HEOM) results for a demanding set of spin-boson models shows that the
interpolation formula has an error comparable to that of RPMD rate theory in
the adiabatic limit, and that of Wolynes theory in non-adiabatic limit, and is
therefore as accurate as any method could possibly be that attempts to
generalise these methods to arbitrary electronic coupling strengths.
