Stephan Kümmel

The construction of consistent pseudopotentials for exchange-correlation approximations that depend on the kinetic energy density [meta-generalized gradient approximations (meta-GGAs)] is challenging because of their explicit orbital dependence. However, modern meta-GGAs use the kinetic energy density to distinguish between one- and many-electron regions of space, and therefore, the straightforward use of inconsistent, e.g., GGA-based pseudopotentials, can lead to substantial errors. We here discuss a scheme for constructing pseudopotentials that incorporate the atomic kinetic energy density in the spirit of a core correction. This considerably increases the accuracy of meta-GGA calculations that use pseudopotentials. Examples and tests are presented for the nonrelativistic and the relativistic Troullier-Martins pseudopotential scheme using real-space grid based calculations. We discuss future extensions of the scheme.

WO3 has many properties that make it a promising anode material for photoelectrochemical water splitting. However, its practical use is limited by its moderate efficiency. Anchoring a co-catalyst to the...

Naphthalenediimide (NDI) is stable under ambient air and an efficient electron acceptor due to its high electron affinity. Phenothiazine derivatives are paradigm electron donors due to their relatively low oxidation potentials and cations of high stability. Combining these two system classes therefore appears as a promising strategy for obtaining a material with attractive optoelectronic properties. We here investigate molecular models of π-coupled junctions of N,N′-bis[3-(triethoxysilyl)propyl]-1,4,5,8-naphthalenediimide (NDI-silane) and 3,7-di-t-butylphenothiazine (TBP) using time-dependent density functional theory. We calculate the electronic excitations for systems with frozen nuclei, and in a second step also investigate the influence that the dynamics of the nuclei has on the electronic excitations. We find optically active excitations around 1.5 eV that are associated with a charge transfer at the interface. We further calculate the electronic couplings between the states that are the most relevant ones for charge separation. Our findings can be seen as indicators for these materials' suitability for photovoltaic applications. First experimental results are in line with the theoretical conclusions.

Antenna complexes absorb sunlight and transfer the harvested energy with remarkable quantum efficiency. In spectroscopic experiments, they are typically excited with laser pulses that differ substantially from sunlight. Using density functional theory calculations in real time, we reveal the excitation dynamics that results in the B850 antenna ring of the purple bacterium Rhodoblastus acidophilus upon excitation by a short, strong pulse as typically used in experiments. The pulse dominantly triggers the exciton modes that are also the most relevant ones in the natural process. Quantum mechanical interference patterns noticeably influence the electronic density distribution after about 40 fs, and on the same time scale, the effects of nuclear motion start to have a noticeable influence on the excitation dynamics. About 20 fs after the laser peak, the B850 ring transitions into dynamics in which the excitation energy is mostly localized on segments that comprise just a few bacteriochlorophyll molecules.

Density functional theory (DFT) is a cornerstone of modern electronic structure theory. In the Kohn–Sham scheme, the many-electron Schrödinger equation is replaced by a set of effective single-particle equations. Thus, the full complexity of the quantum mechanical many-particle effects is mapped to the exchange–correlation potential vxc(r). Almost all DFT calculations done in practice rely on approximations to vxc(r). However, numerical representations of the quasi-exact vxc(r) can be obtained from quasi-exact densities by inverting the Kohn–Sham procedure. This inverse Kohn–Sham scheme is an important source of insight into exact DFT. Here, we review the inverse Kohn–Sham problem and explain in detail several aspects of why Kohn–Sham inversion is intrinsically difficult. We then present several inversion schemes and discuss their pros and cons, specifically addressing the effects of statistical uncertainties that are inevitable in quantum Monte Carlo reference densities. We use these schemes to obtain representations of vxc(r) that correspond to the ground-state densities that have become available from accurate diffusion Monte Carlo calculations on real space grids for the Li2 and N2 molecules, and the C atom. In the latter, the highest occupied orbital has a nodal line and the exchange–correlation potential goes to a different asymptotic value in this direction. As an outlook, we discuss the possibility of interlacing quantum Monte Carlo and Kohn–Sham theory by using the quasi-exact Kohn–Sham determinant to fix the nodes in a diffusion Monte Carlo calculation.

We present techniques that allow for predicting energy transfer in multichromophoric systems based on density-functional-theory calculations in real-time. Our work respects that the time-dependent density is the basic quantity in density-functional theory. In the approach that we discuss here, the simulations are done for a complete multimolecular system, i.e., do not require an a priori decomposition into subsystems. Yet, our analysis tools allow one to reliably extract energy-transfer times between different regions or constituents of the multimolecular system, the structure of transition densities, and the relative degree of excitation of constituents. We demonstrate our approach by analyzing the excitation-energy transfer between six bacteriochlorophyll molecules from the B850 ring of the light-harvesting complex 2 of the purple bacterium Rhodoblastus acidophilus. Our analysis shows that energy is transferred through this system on a time scale of ∼45 fs. The spectral analysis reveals that mainly two supermolecular excitations drive the energy transfer in this system.

We show that the recently proposed Lebeda-Aschebrock-Kümmel (LAK) meta-generalized gradient approximation, the accuracy of which was previously established for atomization energies, bond lengths, and band gaps, also captures weak interactions near equilibrium without a dispersion correction. We discuss how this is achieved. Furthermore, we show that among the semilocal cost pure functionals, LAK is the one that reaches the highest accuracy for the large GMTKN55 database for general thermochemistry and kinetics. Next, we explain the design strategy of enhancement factor engineering. Its key idea is to complement exact constraints with construction principles. Finally, we discuss areas of research in which the use of LAK may offer advantages over existing functionals.

We provide accurate energies and electronic densities for Li2, C, and N2 from the diffusion Monte Carlo (DMC) method in the fixed node approximation based on orbitals from a real-space grid approach. With relatively simple single-determinant trial wave functions, we demonstrate the benefits of an all-electron approach in conjunction with a highly accurate grid method for calculating the orbitals that build the determinant. Our DMC ground state energies match with those of more elaborate single-reference quantum Monte Carlo (QMC) methods based on orbital basis sets. The binning technique is revisited to calculate the electronic density on a spatial grid. We examine the dependence of the resulting mixed estimator and extrapolated density on the trial wave function, specifically on the density functional generating the orbitals, by employing two distinctly different functionals, namely, the local density approximation and the exact-exchange functional. Residual statistical artifacts in the QMC densities are readily corrected by using a regularization method, resulting in smooth densities. As an example for the insight that can be gained from an accurate density, we verify that in the carbon atom, the density along one specific direction can have an asymptotic decay that differs from the decay found in all other directions. We relate this observation to previously published work, which discussed the implications that such a nodal feature may have for the exact Kohn–Sham potential.

The Y6 molecule is one of the most promising non‐fullerene acceptors. Based on first‐principles calculations, this paper analyzes how the separation of an electronic excitation into electron and hole densities is influenced by the interaction between different Y6 molecules. By calculating the optical and the fundamental gaps of ensembles of Y6 molecules that realistically represent a film, their corresponding exciton binding energies are obtained. The combination of range separation, optimal tuning, and dielectric screening endows the presented density functional theory calculations with predictive power. The calculations reveal that the electronic excitations, characterized as electron–hole pairs via their natural transition orbitals, spread across multiple Y6 molecules. A distinct decrease in the exciton binding energy is correlated to a notable charge separation, and the exciton binding energy saturates in ensembles of six to seven Y6 molecules at ≈0.25 electronvolt (eV). These findings contribute to explaining the efficient charge separation in films of Y6. They also give a guideline for the number of molecules that theoretical models should take into account when they aim at a realistic description of charge separation.