The MECP was obtained by a constrained minimization on the crossing seam using a Lagrange multiplier. For the ring-polymer instanton optimization we employed an eigenvector-following algorithm. The optimizations of minima, MECPs and ring-polymer instantons were converged to a maximum total force of 3 • 10 -4 E h A -2 . For the instantons these forces were computed as the negative combined gradient with respect to bead positions and τ of the N -bead ring-polymer potential, U N = SN/ β . The τ -coordinate was scaled by a factor ξ/β with ξ in the range of 5 . . . 15 A. The MRMP gradients and Hessians required for both optimizations and rate calculations were computed by finite differences. Unless otherwise specified, we used atom masses corresponding to the most abundant isotopomer ( 12 C 32 S 35 Cl 2 ).