Importance of Dispersion in the Molecular Geometries of Mn(III) Spin-Crossover Complexes

The
computational investigation of the molecular geometries of
a pair of manganese(III) spin-crossover complexes is reported. For
the geometry of the quintet high-spin state, density functionals significantly
overestimate Mn–Namine bond distances, although
the geometry for the triplet intermediate-spin state is well described.
Comparisons with several wave function-based methods demonstrate that
this error is due to the limited ability of commonly used density
functionals to recover dispersion beyond a certain extent. Among the
methods employed for geometry optimization, restricted open-shell
Møller–Plesset perturbation theory (MP2) appropriately
describes the high-spin geometry but results in a slightly shorter
Mn–O distance in both spin states. On the other hand, extended
multistate complete active space second-order perturbation theory
(XMS-CASPT2) provides a good description of the geometry for the intermediate-spin
state but also sufficiently recovers dispersion, performing well for
the high-spin state. Despite the fact that the electronic structure
of both spin states is dominated by one-electron configuration, XMS-CASPT2
offers a balanced approach, leading to molecular geometries with much
better agreement with experiment than MP2 and DFT. A scan along the
Mn–Namine bond demonstrates that for these complexes
coupled cluster methods (i.e., DLPNO-CCSD(T)) also yield bond distances
in agreement with experiment while multiconfiguration pair density
functional theory (MC-PDFT) is unable to recover dispersion well enough,
analogous to single-reference DFT.