Fig. 2. Analysis of the accuracy of different approximations of the one-mode and two-mode contributions to the structural sensitivity of the calculated XES spectrum of Fe(CO)₅. (a and b) One-mode contributions obtained from calculations for displaced structures with Q₁ = ±4 pm (solid red line) compared to a linearized approximation (solid blue line) and a 3rd order Taylor expansion (dashed green line). The top panels show the corresponding spectra while the lower panels show the change in the calculated spectra. (c and d) Two-mode contributions obtained from calculations for displaced structures with (i.e., |Δq| = 4 pm).