Spin-dependent properties in the framework of the dynamic correlation dressed complete active space method

Abstract

We report an extension of the recently proposed 2nd order dynamic correlation dressed complete active space method [S. Pathak et al., J. Chem. Phys. 147, 234109 (2017)] to incorporate spin-dependent relativistic effects into the Hamiltonian. The result is an effective Hamiltonian that contains the effects of static correlation, dynamic correlation, and relativistic effects on an equal footing. All contributions necessary for the description of magnetic phenomena and electron paramagnetic resonance (EPR) spectroscopy, namely, spin-orbit coupling, magnetic hyperfine coupling, Zeeman interaction, and direct electronic spin-spin coupling, are incorporated. We also suggest a novel analysis of g-matrices and A-matrices based on the singular value decomposition, which can provide not only the magnitude but also the sign of the principal components and allows for a transparent decomposition into different physical contributions. The new method was tested for excitation energies of first-row transition metal ions as well as D-tensors and g-shifts of first-row transition metal complexes using minimal active spaces. It was observed that state-mixing effects are usually small in these cases and that the results are comparable to nondegenerate N-electron valence state perturbation theory (NEVPT2) in conjunction with quasi-degenerate perturbation theory (QDPT). Results on EPR parameters of pseudo-square-planar Cu(ii) complexes show that state-mixing with a ligand-to-metal-charge-transfer configuration greatly improves results compared with NEVPT2/QDPT but also demonstrate that future modifications of the 0th order Hamiltonian or more elaborate electron correlation treatments will be necessary in order to achieve better agreement with the experiment.

Improvement of Ab Initio Ligand Field Theory by Means of Multistate Perturbation Theory

“…Using the gas-phase geometries, this situation is reversed: the first excitation energy is usually underestimated, whereas the second excitation energy is usually closer to experiment. There is a general trend that the NEVPT2 method with triple-zeta basis sets overestimates excitation energies for d−d transitions, as exemplified by our recent study of the excitation energies of free ions in the gas phase, 28 where discrepancies because of an incorrect description of the environment can be excluded. This overestimation of excitation energies was also discussed in the recent AILFT review article.…”

“…25,26 We recently introduced a new multistate multireference perturbation theory (MS-MRPT) method called the secondorder dynamic correlation dressed complete active space method (DCD-CAS(2)). 27,28 Already in our initial work on that method, 27 we observed for one specific complex, [CrF 6 ] 3− , that this new method might provide a better starting point for the AILFT parametrization than the NEVPT2 method that is predominantly used so far to include dynamic correlation. In the present work, we investigate if this finding is part of a more general trend by comparing the performance of DCD-CAS(2) and another recently introduced MS-MRPT (second-order Hermitian quasi-degenerate N-electron valence state perturbation theory, HQD-NEVPT2) 29 when they are combined with AILFT.…”

A DFT study on the mechanism of the formation of 1,4,2,3‐dithiadiazinanes by head‐to‐head [3 + 3] cyclodimerization of thiocarbonylS‐imides

“…CASSCF is a powerful multiconfiguration SCF method which has been used to calculate UV transition as well as spin‐orbit coupling and EPR parameters. [ 44 ] The analysis of effective unpaired electron distributions reveals the radical character of the transition states at the reactive centers C1/N3 and C4/N6 (supporting information Table S2‐6). As expected, 2c‐TS‐I shows significant radical character on C1 and C4 of approximately 36%/11% in both 2 pz ‐orbitals, whereas the N3 and N6 centers bear 32% and 15%, respectively, supporting the asynchronous formation of 2c .…”

Modulating Effect of Ligand Charge on the Electronic Properties of 2Ni–2S Structures and Implications for Biological 2M–2S Sites

“…Additional g tensor and 31 P hyperfine coupling tensor calculations were conducted with the TPSS and B3LYP functionals. , Finally, UV–vis spectra were modeled within the framework of linear response time-dependent DFT (TD-DFT) using the cam-B3LYP range-corrected hybrid functional and employing the Tamm–Dancoff approximation. , The aforementioned DFT calculations were supplemented by a set of complete active space self-consistent field (CASSCF) calculations with five active electrons in four active orbitals that were averaged for the lowest two electronic states. This active space choice was verified for both systems with the recently introduced ASS1ST method. , Magnetic property calculations in this framework were conducted using a quasi-degenerate perturbation theory approach as implemented in ORCA. − The effect of dynamic electron correlation on the total electronic energy of the calculated states was described by means of the strongly contracted variant of n-electron valence perturbation theory. , Like the DFT calculations described above, all multireference calculations employed the def2-TZVP­(-f) basis set and the RI approximation together with the def/J auxiliary basis set to accelerate the required two-electron formation.…”