Electronic and Magnetic Properties of Kremer's tris-Hydroxo Bridged Chromium Dimer: A Challenge for DFT

We present computations of the zero field splitting constants in a tris-hydroxo bridged chromium dimer (Kremer's dimer). A comparison is given of broken symmetry density functional theory (DFT) and multiconfigurational ab initio methods for evaluating ZFS constants. Kremer's dimer is known to be antiferromagnetically coupled, with the spin ladder order of E(S = 0) < E(S = 1) < E(S = 2) < E(S = 3). The B3LYP functional gives the order E(S = 0) < E(S = 3) < E(S = 1) < E(S = 2), and similar results are obtained for other density functionals (PBE, M06, M06-L, and TPSS). In contrast, we find that simple CASSCF calculations yield a correct spin ladder. DFT poorly reproduces the experimental D splitting values, while the CASSCF technique coupled with quasi-degenerate perturbation theory qualitatively reproduces D for all the spin states. State-optimized orbitals result in more accurate spin state energies and D values compared to state-averaged orbitals. Inclusion of spin-spin coupling is found to be essential for both the magnitude and sign of D. The rhombic splitting parameter is found to be near zero, which is comparable to experimental results for which the analysis assumed C3h symmetry.