We investigate a spin-boson inspired model of electron transfer, where the diabatic coupling is given by a position-dependent phase, eiWx. We consider both equilibrium and nonequilibrium initial conditions. We show that, for this model, all equilibrium results are completely invariant to the sign of W (to infinite order). However, the nonequilibrium results do depend on the sign of W, suggesting that photo-induced electron transfer dynamics with spin–orbit coupling can exhibit electronic spin polarization (at least for some time).
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Interestingly, even for a two-state purely real-valued Hamiltonian with a conical intersection, the notion of complex-valued wavefunctions was suggested long ago by Mead and Truhlar.55 The basic premise was that, as you circle a conical intersection, the adiabatic states can be chosen in a well-defined manner by not implementing parallel transport and instead choosing electronic states with a complex-valued phase. This approach has been used by Gherib et al. recently to effectively “isolate” the effect of geometric phase.56 Nevertheless, we must emphasize that, for a two-state real-valued Hamiltonian, there is no Berry force: the Berry force is independent of the phase (or so-called “gauge”) of the electronic adiabats. The Mead–Truhlar mapping in Ref. 55 allows for computational ease without introducing any new physics.