Symmetry Friend or Foe: Confluences of Conical Intersection Seams in Tetra-Atomic Molecules

It is usually assumed that seams of conical intersection are isolated features, that is, a displacement perpendicular to the seam leads to a removal of the degeneracy. However, this need not be the case. It has been shown that for triatomic molecules, there may exist a confluence of an accidental symmetry-allowed C 2 v seam of conical intersection with an accidental C s seam of conical intersection of two states of the same symmetry. Here, we demonstrate the existence of a related confluence of seams of conical intersection in the tetra-atomic molecule isocyanic acid. Such confluences have not been previously reported. A perturbative analysis of the vicinity a seam of conical intersection is used to explain origin of this novel locus of conical intersection points.