Excited electronic states of MnO4−: Challenges for wavefunction and density functional response theories

Abstract

a b s t r a c tThe lowest excited electronic states of the permanganate ion MnO 4 À are calculated using a hierarchy of coupled cluster response approaches, as well as time-dependent density functional theory. It is shown that while full linear response coupled cluster with singles and doubles (or higher) performs well, that permanganate represents a stern test for approximate coupled cluster response models, and that problems can be traced to very large orbital relaxation effects. TD-DFT is reasonably robust although errors around 0.6 eV are still observed. In order to further investigate the strong correlations prevalent in the electronic ground state large-scale RASSCF calculations were also performed. Again very large orbital relaxation in the correlated wavefunction is observed. Although the system can qualitatively be described by a single configuration, multi-reference diagnostic values show that care must be taken in this and similar metal complexes.

Applications of Time-Dependent and Time-Independent Density Functional Theory to Electronic Transitions in Tetrahedral d0 Metal Oxides

“…61 We compare in Table 1 the calculated excitation energies for the first three dipole allowed transitions to singlet T 2 states with the experiment. For the variational methods (HF, CISD, SAC-CI, RAS-PT2), we note a clear convergence toward the experimental values in going from the simplest (HF) to the most extensive ab initio scheme (RAS-PT2).…”

One‐ and Two‐Photon‐Induced Photochemistry of Iron Pentacarbonyl [Fe(CO)5]: Insights from Coupled Cluster Response Theory

“…The optimised D 3h geometry of trigonal bipyramid Fe(CO) 5 is shown in Figure 1 agrees favourably with a range of experimental geometries [40,41] leading to the conclusion that the CCSD optimised geometries perform well and that any effects caused by differences in geometry should be minimal, a property observed in the study of other, similar species. [30,42,43] A lack of accuracy caused by the size or quality of the basis set used should not be present here as a basis set of equivalent size was applied to a similar system elsewhere [19] for the one photon absorption spectrum of Cr(CO) 6 with no negative effects on the quality of the results. Correlation effects are also very high in this complex, evident in the mixed nature of the states presented in Table 1.…”

“…This has been observed previously for CC2 in certain transition metal compounds, including TiO 2 , [43] Ni(CO) 4 , [42] and MnO 4 À . [30] This problem, sometimes observed for CC2, for transition metal systems has been attributed to the large T 1 amplitudes which are used as similarity transform operators to ensure that the singles are treated at zeroth order both with and without the external field perturbation. [30,44] !eq π*)/ (3d x 2 À y 2 � !eq π*) 4.775 0.0000…”

“…This system will add to the few benchmarks available in this area. [20,30] Furthermore, CC response theory readily allows for non-linear optical properties such as two-photon absorption, [29] for which some unassigned experimental data exists for Fe(CO) 5 .…”

Investigation of challenging spin systems using Monte Carlo configuration interaction and the density matrix renormalization group

“…First‐row monocarbonyls are known to be challenging to study computationally; energy level proximity and multireference character can lead to significant errors for traditional approaches, for example, NiCO . The complex nature of the electronic spectra and a high density of states presents a similar challenge as observed for metal oxides, for example, TiO 2 and MnO , where standard response methods may fail in describing excited states due to problems appropriately describing the ground state.…”