To test our approach, we chose [Fe(TPP)(2-MeHIm)], see Figure ), which is an Fe(II) high spin complex. This is advantageous because Fe(II) high spin complexes have exceptionally high isomer shifts which are easily discernible from other spin and oxidation states. Its experimental Mössbauer parameters are taken from Ref. , see Table for the pronounced temperature dependence of its quadrupole splitting. While iron is notorious for its complicated electronic structure, which often renders it challenging for DFT as a singledeterminant method, the large number of single point calculations for the ensemble required DFT as an efficient computational method. The high spin state of [Fe(TPP)(2-MeHIm)] was identified as the ground state (see Supplementary Table for calculated spin state energies). The Mössbauer parameters for the equilibrium structure are δ = 0.94 mm s -1 and ∆E Q = 2.84 mm s -1 in good to reasonable agreement with the experimental findings. The Mössbauer parameters of all structures in the generated ensembles at 77K, 195K and 300K are shown in Figure . For the isomer shift, no temperature dependence is observed in agreement with experiment and expectations. Strikingly, for the quadrupole splitting we not only observe the expected scatter around the positive value for the equilibrium structure, but in addition that some members of the ensemble have negative quadrupole splitting values. Moreover, the number of species with negative quadrupole splitting values increases with increasing temperature. We thus conclude that a key contributor to the temperature dependence of the quadrupole splitting are vibrational distortions that -crucially-lead to a swap in the sign of the quadrupole splitting. Note that experimentally, only the absolute value of the quadrupole splitting is routinely measured. Indeed, averaging the quadrupole splitting value over the ensemble, see Table and Figure , qualitatively reproduces the experimental trend of a decreasing quadrupole splitting with higher temperature. Table Experimental and calculated Mössbauer parameters of [Fe(TPP)(2-MeHIm)] at different temperatures; in "calculations (all)", the Mössbauer parameters are averaged over all members of the ensemble, in "calculations (filtered)" the Mössbauer parameters are only averaged over structures with a d x 2 -y 2 (β ) orbital occupation < 32 %.