  We propose a new quantum transition-state theory for calculating Fermi's
golden-rule rates in complex multidimensional systems. This method is able to
account for the nuclear quantum effects of delocalization, zero-point energy
and tunnelling in an electron-transfer reaction. It is related to instanton
theory but can be computed by path-integral sampling and is thus applicable to
treat molecular reactions in solution. A constraint functional based on energy
conservation is introduced which ensures that the dominant paths contributing
to the reaction rate are sampled. We prove that the theory gives exact results
for a system of crossed linear potentials and also the correct classical limit
for any system. In numerical tests, the new method is also seen to be accurate
for anharmonic systems, and even gives good predictions for rates in the Marcus
inverted regime.
