6 Search Results

The high-energy shoulder in the gas-phase fluorescence emission spectrum of pyrene is a well-known example of non-Kasha emission. Here, we comparatively assess two approaches, vibronic perturbation theory and nonadiabatic dynamics, in their ability to predict and explain the gas-phase fluorescence spectrum of pyrene. While both methods qualitatively capture the non-Kasha emission, they differ in their computational requirements, accuracy, and physical interpretation. Vibronic perturbation theory and nonadiabatic dynamics are complementary and can be combined in a two-step approach to non-Kasha fluorescence.

It has been known for more than a decade that the gauge variance of the kinetic energy density τ leads to additional terms in the magnetic orbital rotation Hessian used in linear-response time-dependent density functional theory (TDDFT), affecting excitation energies obtained with τ-dependent exchange–correlation functionals. While previous investigations found that a correction scheme based on the paramagnetic current density has a small effect on benchmark results, we report more pronounced effects here, in particular, for the popular M06-2X functional and for some other meta-generalized gradient approximations (mGGAs). In the first part of this communication, this is shown by a reassessmentmore » of a set of five Ni(II) complexes for which a previous benchmark study that did not impose gauge invariance has found surprisingly large errors for excitation energies obtained with M06-2X. These errors are more than halved by restoring gauge invariance. The variable importance of imposing gauge invariance for different mGGA-based functionals can be rationalized by the derivative of the mGGA exchange energy integrand with respect to τ. In the second part, a large set of valence excitations in small main-group molecules is analyzed. For M06-2X, several selected n → π* and π→π$$^{*}_{⊥}$$ excitations are heavily gauge-dependent with average changes of –0.17 and –0.28 eV, respectively, while π→π$$^{*}_{∥}$$ excitations are marginally affected (–0.04 eV). Similar patterns, but of the opposite signs, are found for SCAN0. Here, the results suggest that reevaluation of previous gauge variant TDDFT results based on M06-2X and other mGGA functionals is warranted.« less

Diaminoquinones with a captodatively stabilized biradicaloid structure are options for singlet fission, but few such compounds are known. We report the solution spectroscopy and photophysics of 1,2,2,3-tetramethyl-2,3-dihydro-1H-naphtho[2,3-d]imidazole-4,9-dione (1): its steady-state and transient UV-visible absorption, linear dichroism in stretched poly(vinyl alcohol), and magnetic circular dichroism. We also describe the absorption spectra of the stable radical ions 1+ and 1- and of two parent structures, 2,3-diamino-1,4-naphthoquinone (2) and 2,3-bis(methylamino)-1,4-naphthoquinone (3). The spectra are interpreted and electronic transitions are assigned by comparison with the results of density functional theory and MS-CASPT2 calculations.

TURBOMOLE is a collaborative, multi-national software development project aiming to provide highly efficient and stable computational tools for quantum chemical simulations of molecules, clusters, periodic systems, and solutions. The TURBOMOLE software suite is optimized for widely available, inexpensive, and resource-efficient hardware such as multi-core workstations and small computer clusters. TURBOMOLE specializes in electronic structure methods with outstanding accuracy–cost ratio, such as density functional theory including local hybrids and the random phase approximation (RPA), GW-Bethe–Salpeter methods, second-order Møller–Plesset theory, and explicitly correlated coupled-cluster methods. TURBOMOLE is based on Gaussian basis sets and has been pivotal for the development of many fastmore » and low-scaling algorithms in the past three decades, such as integral-direct methods, fast multipole methods, the resolution-of-the-identity approximation, imaginary frequency integration, Laplace transform, and pair natural orbital methods. This review focuses on recent additions to TURBOMOLE’s functionality, including excited-state methods, RPA and Green’s function methods, relativistic approaches, high-order molecular properties, solvation effects, and periodic systems. A variety of illustrative applications along with accuracy and timing data are discussed. Moreover, available interfaces to users as well as other software are summarized. TURBOMOLE’s current licensing, distribution, and support model are discussed, and an overview of TURBOMOLE’s development workflow is provided. Challenges such as communication and outreach, software infrastructure, and funding are highlighted.« less

Chromophores suitable for singlet fission need to meet specific requirements regarding the relative energies of their S₀, S₁, and T₁ (and T₂) electronic states. Accurate quantum-chemical computations of the corresponding energy differences are thus highly desirable for materials design. Methods based on density functional theory (DFT) have the advantage of being applicable to larger, often more relevant systems compared to more sophisticated post-Hartree–Fock methods. However, most exchange–correlation functionals do not provide the needed accuracy, in particular, due to an insufficient description of the T₁ state. Here we use a recent singlet fission chromophore test set (Wen, J.; Havlas, Z.; Michl,more » J. J. Am. Chem. Soc. 2015, 137, 165-172) to evaluate a wide range of DFT-based methods, with an emphasis on local hybrid functionals with a position-dependent exact-exchange admixture. New reference vertical CC2/CBS benchmark excitation energies for the test set have been generated, which exhibit somewhat more uniform accuracy than the previous CASPT2-based data. These CC2 reference data have been used to evaluate a wide range of functionals, comparing full linear-response TDDFT, the Tamm–Dancoff approximation (TDA), and ΔSCF calculations. Two simple two-parameter local hybrid functionals and the more empirical M06-2X global meta-GGA hybrid provide the overall best accuracy. Due to its lower empiricism and wide applicability, the Lh12ct-SsifPW92 local hybrid is suggested as the main ingredient of an efficient computational protocol for prediction of the relevant excitation energies in singlet fission chromophores. Full TDDFT for the S₁, S₂, and T₂ excitations is combined with ΔSCF for the T1 excitations. Making use also of some error compensation with suitable DFT-optimized structures, even the most critical T1 excitations can be brought close to the target accuracy of 0.20 eV, while the other excitation energies are obtained even more accurately. This fully DFT-based protocol should become a useful tool in the field of singlet fission.« less