  Geometric phases play a crucial role in diverse fields. In chemistry they
appear when a reaction path encircles an intersection between adiabatic
potential energy surfaces and the molecular wavefunction experiences
quantum-mechanical interference effects. This intriguing effect, closely
resembling the magnetic Aharonov-Bohm effect, crucially relies on the adiabatic
description of the dynamics, and it is uncertain whether and how it persists in
an exact quantum dynamical framework. Recent works have shown that the
geometric phase is an artifact of the adiabatic approximation, thereby
challenging the perceived utility of the geometric phase concept in molecules.
Here, we investigate this issue in an exact dynamical framework. We introduce
instantaneous, gauge invariant phases separately for the electrons and for the
nuclei, and use them to monitor the phase difference between the trailing edges
of a wavepacket encircling a conical intersection. In this way we unambiguosly
assess the role of the geometric phase in the interference process and shed
light on its persistence in molecular systems.
