On the noncrossing rule in polyatomic systems: Determination of a seam of actual surface crossings relevant to the quenching of H2(B 1Σ+u) by helium

A recently developed method for determining avoided surface crossings using analytic gradient techniques is used to locate an actual crossing seam for the 1 1A′ and 2 1A′ potential energy surfaces of the He–H2 system. This seam is not related to any high symmetry nuclear configurations. The computational procedure, which is based on the minimization of ΔEIJ(R)2≡[EI(R)−EJ(R)]2, &(R)]2, uses different density matrices to simplify the construction of the energy difference gradient, the most costly step in the procedure. The actual crossing seam, R(r), is specified by the ordered triple R(r)≡[R(r), γ(r),r] for which ΔEIJ(R)=0. It is exoergic with respect to the He–H2(B 1Σ+u) asymptote for r≊[2.60, 5.70]. Here r≡R(H2), R=R(He–H2) and γ is the He–H2 angle. This seam defines a region nuclear coordinate space near which helium can efficiently quench H2(B 1Σ+u).