Born–Oppenheimer Conical Intersection

At a conical intersection, two Born–Oppenheimer surfaces touch at a point in nuclear coordinate space. The upper/lower adiabatic surfaces E_± = H₁₁+H₂₂/2 ± √((H₁₁-H₂₂)²+4H₁₂²)/2 form a double cone. A wavepacket encircling the CI acquires a geometric Berry phase of π.