A numerical issue arises with the (two) first and (two) last trajectories at the edges of the discrete C i -grid ensemble, since these lack a sufficient number of neighbors to evaluate spatial derivatives using the finite-difference scheme. In reality, this is nothing but the aforementioned boundary condition difficulty. This problem is mitigated by introducing additional fixed virtual trajectories x 0 and x -1 set at -∞, and x n+1 and x n+2 at +∞. Note that (x 0x -1 ) = (x n+2x n+1 ) = +∞ is also presumed. With these choices, reasonable (i.e., non-singular) values for F Q are obtained using Eq. ( )-although as discussed, errors can still propagate in from the edges, particularly if there is a scattering PES present.