For well-behaved cases, MECP can be easily determined by optimization of the seam. We were able to locate MECP for acetylene-and propyne-derived diradicals by using this technique with and without constraint of the symmetry plane of the triplet geometries. To verify that the found stationary crossing points are minima, we performed frequency analysis along the seam (this procedure is analogous to the verification of minima versus saddle points in geometry optimization). We found that the symmetric structures have an out-of-plane imaginary mode of the effective Hessian (see Fig. in SI). The frequencies at MECP for acetylene-and propynederived diradicals are given in SI. As expected from MECP properties (see Section 4 in SI), the gradients at the optimized MECP geometries are almost collinear. All angles are close to 180 • , corresponding to the crossing of two parabolas, with separated minima and crossing in between these minima. The deviation of the angles between the gradients is larger for the propynederived diradical than for the acetylene-derived diradical, likely because of a larger number of degrees of freedom. For acetylene-and propyne-derived systems, the MECP geometries are close to the equilibrium geometry of the singlet diradical. Consequently, the MECP and the singlet minima are also close energetically (see Table in the SI). Another observation is that the contribution of singlet Hessian dominates the effective Hessian and frequencies of MECP are singlet-like (Table in SI).