arXiv subjects

Konrad Merkel

Publications and source records attributed to Konrad Merkel.

4 recordsLinked to original sources

arXiv2026

Interplay between Electronic Structure, Chemical Bonding, and Lattice Symmetry in Bismuth Vanadate

Bismuth vanadate (BiVO$_4$) is a prototypical oxide photocatalyst that occurs in both tetragonal and monoclinic scheelite phases with markedly different photocatalytic and photoelectrochemical activities. Accurately identifying the monoclinic phase as the ground state and explaining the origin of its symmetry-breaking distortion are unusually challenging from a theoretical perspective, with various levels of theory and associated physical interpretations for this behaviour reported in the literature. Here, we resolve these discrepancies by systematically assessing the role of exact exchange with and without spin-orbit coupling, demonstrating that an accurate treatment of electronic localization is essential to stabilize the monoclinic scheelite structure. Using this framework, we compute the electronic band structure through dense sampling of the Brillouin zone and show that the band edges in monoclinic and tetragonal BiVO$_4$ lie far from conventional high-symmetry paths, leading to substantial differences in band gaps and carrier effective masses. Choosing the exchange-correlation functional that best reproduces the crystal structure leads to excellent predictions of the band gap once excitonic and thermal effects are taken into account. In addition, we show that the monoclinic distortion is driven by charge transfer between non-equivalent oxygen sites, which breaks the lattice symmetry and is suppressed by self-interaction errors when using semi-local DFT. These results establish a direct connection between the exchange-correlation functional, electronic localization, chemical bonding, and structural stability in BiVO$_4$, providing a foundation for robust ab initio descriptions of phase stability and optoelectronic properties in such complex oxides.

cond-mat.mtrl-sci↗

arXiv2025

Evaluating First-Principles Electron-Phonon Couplings: Consistency Across Methods and Implementations

Electron-phonon coupling (EPC) is fundamental for understanding the behavior of molecules and crystals, influencing phenomena such as charge transport, energy transfer, phase transitions, and polaron formation. Accurate computational methods to calculate EPCs from first principles are essential, but their complexity has resulted in a variety of computational strategies, raising concerns about their mutual consistency. In this study, we provide a systematic benchmark of methods for EPC calculation by comparing two fundamentally different {\it ab initio} methodologies. We investigate Gaussian-type orbital methods based on the \textsc{CP2K} code and plane-wave-based projector-augmented-wave (PAW) methods combined with maximally localized Wannier functions, as implemented in \textsc{VASP} and \textsc{wannier90}. In addition, we further distinguish between the derivative--of--Hamiltonian ($dH$) and derivative--of--states ($d\psi$) approaches for obtaining EPC parameters. The comparison is conducted on a representative set of organic molecules, including pyrazine, pyridine, bithiophene, and quarterthiophene, varying significantly in size and flexibility. We find excellent agreement across implementations and basis sets when employing the same computational approach ($dH$ or $d\psi$), demonstrating robust consistency between the numerical schemes. However, noticeable deviations occur when comparing the $dH$ and $d\psi$ approaches within each code and for specific cases discussed in detail. Our findings emphasize the reliability of EPC computations using the $dH$ method and caution against potential pitfalls associated with the $d\psi$ approach, providing guidance for future EPC calculations and model parameterizations.

physics.chem-ph↗

arXiv2023

Linear Scaling Approach for Optical Excitations Using Maximally Localized Wannier Functions

We present a theoretical method for calculating optical absorption spectra based on maximally localized Wannier functions, which is suitable for large periodic systems. For this purpose, we calculate the exciton Hamiltonian, which determines the Bethe-Salpeter equation for the macroscopic polarization function and optical absorption characteristics. The Wannier functions are specific to each material and provide a minimal and therefore computationally convenient basis. Furthermore, their strong localization greatly improves the computational performance in two ways: first, the resulting Hamiltonian becomes very sparse and, second, the electron-hole interaction terms can be evaluated efficiently in real space, where large electron-hole distances are handled by a multipole expansion. For the calculation of optical spectra we employ the sparse exciton Hamiltonian in a time-domain approach, which scales linearly with system size. We demonstrate the method for bulk silicon - one of the most frequently studied benchmark systems - and envision calculating optical properties of systems with much larger and more complex unit cells, which are presently computationally prohibitive.

cond-mat.mes-hall↗

arXiv2020

Phase space theory for open quantum systems with local and collective dissipative processes

In this article we investigate driven dissipative quantum dynamics of an ensemble of two-level systems given by a Markovian master equation with collective and non-collective dissipators. Exploiting the permutation symmetry in our model, we employ a phase space approach for the solution of this equation in terms of a diagonal representation with respect to certain generalized spin coherent states. Remarkably, this allows to interpolate between mean-field theory and finite system size in a formalism independent of Hilbert-space dimension. Moreover, in certain parameter regimes, the evolution equation for the corresponding quasiprobability distribution resembles a Fokker-Planck equation, which can be efficiently solved by stochastic calculus. Then, the dynamics can be seen as classical in the sense that no entanglement between the two-level systems is generated. Our results expose, utilize and promote techniques pioneered in the context of laser theory, which we now apply to problems of current theoretical and experimental interest.

quant-ph↗