Tetra-radical and ionic S1/S0 conical intersections of cyclobutadiene

Ionic conical intersection (CIionic) and tetra-radical conical intersection (CItetra) between the S1 state and S0 state of cyclobutadiene have been located. While CIionic is involved in the automerization, CItetra is involved in not only the automerization but also the criss-cross reaction.