Systematic location of intersecting seams of conical intersection in triatomic molecules: The 1 ²A′-2 ²A′ conical intersections in BH₂

Points of conical intersection are continuously connected forming seams. Recently a quite unanticipated situation has been found in which two distinct seams of conical intersection - one symmetry-allowed and one same-symmetry - originating from the same two states intersect each other. The identification of these confluences, based on ab initio electronic wave functions has been somewhat serendipitous. A systematic approach for locating such confluences, based solely on information obtained on the symmetry-allowed portion of the seam, has been suggested. In this work that approach is applied to identify the point where a C_s seam of conical intersection intersects a symmetry-allowed C_2v seam of conical intersection for the 1 ²A′ and 2 ²A′ states of BH₂, states that correlate with B(1s²2s²2p,²P)+ H₂. It is suggested, based on this and previous work, that this unexpected situation, which has fundamental implications for our understanding of nonadiabatic processes, is not at all uncommon.