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We present an extension of the combined density functional theory (DFT) and multireference configuration interaction (MRCI) method (DFT/MRCI) [S. Grimme and M. Waletzke, J. Chem. Phys. 111, 5645 (1999)] for the calculation of core-excited states based on the core-valence separation (CVS) approximation. The resulting method, CVS-DFT/MRCI, is validated via the simulation of the K-edge X-ray absorption spectra of 40 organic chromophores, amino acids, and nucleobases, ranging in size from CO2 to tryptophan. Overall, the CVS-DFT/MRCI method is found to yield accurate X-ray absorption spectra (XAS), with consistent errors in peak positions of ∼2.5–3.5 eV. Additionally, we show that the CVS-DFT/MRCI method may be employed to simulate XAS from valence excited states and compare the simulated spectra to those computed using the established wave function-based approaches [ADC(2) and ADC(2)x]. In general, each of the methods yields excited state XAS spectra in qualitative and often quantitative agreement. In the instances where the methods differ, the CVS-DFT/MRCI simulations predict intensity for transitions for which the underlying electronic states are characterized by doubly excited configurations relative to the ground state configuration. Here, we aim to demonstrate that the CVS-DFT/MRCI approach occupies a specific niche among numerous other electronic structure methods in this area, offering the ability to treat initial states of arbitrary electronic character while maintaining a low computational cost and comparatively black box usage. [ABSTRACT FROM AUTHOR]

The combination of density functional theory and multireference configuration interaction (DFT/MRCI) is a well-established semi-empirical method suitable for computing spectral properties of large molecular systems. To this day, three different Hamiltonians and various parameter set combinations exist. These DFT/MRCI variants are well tried and tested when it comes to electronic excitations of organic molecules. For transition metal complexes, systematic benchmarks against experimental data are missing, however. Here we present an assessment of the DFT/MRCI variants and of time-dependent, linear-response density functional theory (TDDFT) for a diverse set of ligand-centered, metal-to-ligand charge transfer, metal-centered, and ligand-to-metal charge transfer (LMCT) excitations on 21 3d and 4d complexes comprising 10 small inorganic and 11 larger metalorganic compounds with closed-shell ground states. In the course of this assessment, we realized that the excitation energies of transition metal complexes can be very sensitive with respect to the details of the damping function that scales off-diagonal matrix elements. This scaling is required in DFT/MRCI to avoid double counting of dynamic electron correlation. These insights lead to a new Hamiltonian, denoted R2018, with improved performance on transition metal compounds, while the results for organic molecules are nearly unaffected by the modified damping function. Two parameter sets were optimized for this Hamiltonian: One set is to be used in conjunction with the standard configuration selection threshold of 1.0 Eh and a second set is for use with a selection threshold of 0.8 Eh which leads to shorter wave function expansions. The R2018 Hamiltonian in standard parameterization achieves root-mean-square errors (RMSEs) of merely 0.15 eV for the metalorganic complexes, followed by 0.20 eV for the original DFT/MRCI ansatz, and 0.25 eV for the redesigned DFT/MRCI approach. In comparison, TDDFT gives a much larger RMSE of 0.46 eV for metalorganic complexes. None of the DFT/MRCI variants yields convincing results for small oxides and fluorides which exhibit LMCT transitions. Here, TDDFT performs better. If the oxides and fluorides are excluded from the inorganic test set, satisfactory agreement can be achieved, with RMSE values between 0.26 eV and 0.30 eV for DFT/MRCI and 0.34 eV for TDDFT. The performance of the original and the new DFT/MRCI Hamiltonians deteriorates only slightly, when a tighter selection threshold is chosen, thus enabling the computation of reliable spectral properties even for large metalorganic complexes. [ABSTRACT FROM AUTHOR]

DFT/MRCI is a well-established method of Grimme and Waletzke [J. Chem. Phys. 111, 5645 (1999)] combining density functional theory and multireference configuration interaction. It was later redesigned by Lyskov, Kleinschmidt, and Marian [J. Chem. Phys. 144, 034104 (2016)] to provide a better treatment of bi-chromophores while treating all other systems as well as Grimme's version did by computing individual energy shifts for each state function of a configuration. But all previous operators lack the ability to compute states with an odd number of electrons (doublet and quartet states). Here we present a general Hamiltonian based on Lyskov's redesign which calculates excited singlet, doublet, triplet, and quartet states of systems that have up to one open shell in the parent determinant. The multiplicity-independent correction parameters provide an extra correction for the open shell in the parent determinant. The Hamiltonian in combination with two parameter sets for different selection thresholds has been tested and compared to experimental vertical excitation and ionization energies yielding similar statistics for all multiplicities with a root mean square deviation smaller than 0.2 eV while maintaining the good computational performance of the Hamiltonians of Grimme and Lyskov. [ABSTRACT FROM AUTHOR]

Stable radical-based fluorescent emitters are rare, even more so at room temperature. The zinc(II) tripyrrindione [Zn(TD1˙)(H2O)] complex has recently been described experimentally by Gautam et al. [Inorg. Chem., 2018, 57, 15240] as a new member of the small family of neutral radical emitters with possible applications in electronics and photonics. Upon excitation at the absorption maximum of 599 nm (2.07 eV), strong fluorescence was observed with a maximum at 644 nm (1.93 eV) at room temperature in tetrahydrofuran solution. The fluorescence energy is higher than several low-intensity absorption bands starting at ≈930 nm. Here we present a theoretical investigation into the absorption and fluorescence of this zinc complex by means of the recently developed semi-empirical all-multiplicity DFT/MRCI-R2018 method. The DFT/MRCI method combines density functional theory (DFT) in a closed shell or restricted open-shell Kohn–Sham orbital basis and multireference configuration interaction (MRCI). The R2018 Hamiltonian proves to be well-suited for investigating the properties of the radical-based zinc complex. The calculations reveal that the absorption spectrum is dominated by bright transitions to the D3, D6 and D11 states. The experimentally observed emission band lies at considerably shorter wavelengths than the lowest absorption band of the radical. This precludes the D1 → D0 transition as the origin of the emission. Our calculations indicate a non-Kasha emission, with D3 as the emissive state. Other ways of explaining the experimentally observed emission, such as ion-pair formation or ligand emission after demetalation, are discussed as well. [ABSTRACT FROM AUTHOR]

In this work, we present a detailed analysis of the photophysical properties of four phosphorescent iridium(III) complexes, i.e.trans-N,N- andcis-N,N-(ppy)2IrIII(acac) as well as their fluorinated derivativestrans-N,N- andcis-N,N-(F2ppy)2IrIII(acac). These properties include absorption and emission characteristics, intersystem crossing rates from the lowest singlet excited state, phosphorescence lifetimes of the individual triplet sublevels as well as the orientations of the transition dipole vectors. To this end, we have carried out combined density functional theory and multi-reference configuration interaction studies including spin–orbit coupling by perturbational as well as variational procedures. For the experimentally known complexes, we observe excellent agreement between our computed data and literature data. Also the blueshifts of the emission maxima occurring upon fluorination of the (ppy)2Ir(acac) compounds are well reproduced. To our surprise, we find the experimentally not yet investigatedcis-N,N-(F2ppy)2Ir(acac) isomer to be thermodynamically more stable than the well-known blue phosphorescent emittertrans-N,N-(F2ppy)2Ir(acac). [ABSTRACT FROM AUTHOR]

In the past two decades, the combined density functional theory and multireference configuration interaction (DFT/MRCI) method has developed from a powerful approach for computing spectral properties of singlet and triplet excited states of large molecules into a more general multireference method applicable to states of all spin multiplicities. In its original formulation, it shows great efficiency in the evaluation of singlet and triplet excited states which mainly originate from local one‐electron transitions. Moreover, DFT/MRCI is one of the few methods applicable to large systems that yields the correct ordering of states in extended π‐systems where double excitations play a significant role. A recently redesigned DFT/MRCI Hamiltonian extends the application range of the method to bi‐chromophores such as hydrogen‐bonded or π‐stacked dimers and loosely coupled donor–acceptor systems. In conjunction with a restricted‐open shell Kohn–Sham optimization of the molecular orbitals, even electronically excited doublet and quartet states can be addressed. After a short outline of the general ideas behind this semi‐empirical method and a brief review of alternative approaches combining density functional and multireference wavefunction theory, formulae for the DFT/MRCI Hamiltonian matrix elements are presented and the adjustments of the two‐electron contributions are discussed. The performance of the DFT/MRCI variants on excitation energies of organic molecules and transition metal compounds against experimental or ab initio reference data is analyzed and case studies are presented which show the strengths and limitations of the method. Finally, an overview over the properties available from DFT/MRCI wavefunctions and further developments is given. This article is categorized under: Electronic Structure Theory > Density Functional TheoryElectronic Structure Theory > Semiempirical Electronic Structure MethodsSoftware > Quantum Chemistry The DFT/MRCI method employs a few empirically adjusted global parameters for modifying the CI Hamiltonian matrix elements. Benchmark calculations attest to its remarkably good performance for excited‐state energies and other spectral properties at moderate computational expense. [ABSTRACT FROM AUTHOR]