Communication: Machine learning of double-valued nonadiabatic coupling vectors around conical intersections

In recent years, machine learning has had an enormous success in fitting ab initio potential-energy surfaces to enable efficient simulations of molecules in their ground electronic state. In order to extend this approach to excited-state dynamics, one must not only learn the potentials, but also nonadiabatic coupling vectors (NACs). There is a particular difficulty in learning NACs in systems which exhibit conical intersections, as due to the geometric-phase effect, the NACs may be double-valued, and are thus not suitable as training data for standard machine-learning techniques. In this work, we introduce a set of auxiliary single-valued functions from which the NACs can be reconstructed, thus enabling a reliable machine-learning approach.