to obtain cubic and quartic anharmonic corrections. This approach has been used in a series of Fe(II) spin-crossover complexes. The harmonic potential is included for comparison where b and c are set to zero and a = 1 2 ω i where ω i is the harmonic vibrational frequency (Figure ). Significant anharmonicity is observed in both complexes. The fitting results in an very large negative value for b due to the significant opening of the potential on the right hand side, as well as a large positive The anharmonic frequency can be obtained from the quadratic term and is 272 cm -1 for Cr 2 (mhp) 4 and 333 cm -1 for Cr 2 (mhp) 4 . The inclusion of anharmonic corrections shifts the vibrational mode to a lower value compared to the harmonic mode by 10 and 20 cm -1 , respectively. The CASPT2 minima obtained from the PES scan of Cr 2 (mhp) 4 without the IPEA shift is the same as that obtained by geometry optimization, 1.989 Å. This is true using either the Dunning or the ANO-RCC basis sets. However, the minima shifted to 1.887 Å when the IPEA shift is employed, in excellent agreement with the experimental value of 1.889(1) Å (Table , Figures and). These results are consistent with work by Ruiperez 4 , respectively. The bonding in the three complexes is weaker than their formal assignments based on bond distance and consistent with lower frequency vibrational modes. The complexity of the potential energy surface around the Cr centers manifests in this rich low-energy spectrum, in particular for Cr 2 (mhp) 4 and Cr 2 (dmp) 4 .