  Ab initio modeling of conical intersection dynamics is crucial for various
photochemical, photophysical, and biological processes. However, adiabatic
electronic states obtained from electronic structure computations involve
random phases, or more generally, random gauge fixings, which hampers the
modeling of nonadiabatic molecular dynamics. Here we develop a random-gauge
local diabatic representation that allows an exact modeling of conical
intersection dynamics directly using the adiabatic electronic states with
phases randomly assigned during the electronic structure computations. Its
utility is demonstrated by an exact ab initio modeling of the two-dimensional
Shin-Metiu model with and without an external magnetic field. Our results
provide a simple approach to integrating the electronic structure computations
into non-adiabatic quantum dynamics, thus paving the way for ab initio modeling
of conical intersection dynamics.
