Conical intersections (CIs) occur when adiabatic potential energy surfaces (PES)  intersect. These points provide the ability for a system to undergo rapid diabatic relaxations from one energy surface to another. Consequently, they also fall outside of the Born-Oppenheimer regime. Conical intersections are important to understanding chemical dynamics, such as in the case relaxation from an excited to a ground state occurs. Rotation in configuration space around a CI point introduces a phase shift in the electronic wavefunction, known as the Berry phase. Advanced femtosecond optical spectroscopy has been able to show experimentally the role CIs play in chemical dynamics. 

Application uses the Maple Quantum Chemistry Toolbox.