Equation ( ) is thus unique up to a gauge encoded in a phase factor e (i/h)θ(x,t) (with θ(x, t) a real function): multiplying the nuclear wavefunction by this factor and the electronic term by its complex conjugate, Eq. (3) remains unaffected and the time-dependent potentials transform as standard gauge potentials. Therefore, this ambiguity has to be eliminated by imposing a choice of gauge.