arXiv subjects

Roland Mitric

Publications and source records attributed to Roland Mitric.

At least 19 recordsLinked to original sources

arXiv2026

FMO-xTB: Fragment molecular orbital method with GFN1-xTB for large-scale quantum-mechanical simulations

We present the fragment molecular orbital method (FMO) combined with the GFN1-xTB extended tight-binding approach (FMO-xTB) for efficient quantum-mechanical calculations of large molecular systems. Both the two-body (FMO2) and three-body (FMO3) expansions are formulated, and fully analytic energy gradients including the response contribution from the self-consistent embedding potential are derived and implemented. The FMO-xTB method inherits the broad element coverage of GFN1-xTB, which employs element-specific rather than atom-pair-specific parameters and is parameterized for all spd-block elements up to radon(Z = 86), representing a significant practical advantage over FMO- DFTB approaches. The accuracy of FMO-xTB is systematically benchmarked against non-fragmented xTB calculations for water clusters, anthracene aggregates, and pentacene supercells. FMO3-xTB reproduces the reference energies with deviations on the order of 10^-4 Hartree for organic semiconductor systems. The covalent bond fragmentation capability using the hybrid orbital projection (HOP) boundary treatment is also implemented with fully analytic gradients and validated for polyalanine alpha-helices and B-DNA double helices, yielding FMO3-xTB energy deviations on the order of 10^-6 Hartree for polyalanine and in the millihartree range for B-DNA. Near-linear scaling is achieved with effective scaling exponents between b= 1.06 and b= 1.28, compared to cubic scaling for non-fragmented xTB. Parallelization over multiple CPU cores yields significant speed ups, and a complete energy and gradient evaluation of a pentacene supercell containing 23760 atoms is feasible within minutes on a single computing node, enabling routine molecular dynamics simulations of systems with tens of thousands of atoms. The method is implemented in the DIALECT software package.

physics.chem-ph↗

arXiv2026

Exciton-Exciton and Exciton-Photon Annihilation in Polaritonic Systems

Strong light--matter interactions forming hybrid quasiparticles termed polaritons can specifically tailor molecular photophysics. In this spirit, enhancing energy transport has recently been of special interest. Exciton--exciton annihilation is commonly used to quantify energy transfer in excitonic systems, and has been recently applied to investigate transport dynamics in polaritonic systems. However, the interpretation of experimental findings is challenging without a microscopic theory that accounts for the various nonradiative relaxation channels determining the quasiparticle diffusion length. In this work, we develop a microscopic model for polariton annihilation based on exciton--exciton annihilation and propose an exciton--photon annihilation as the decisive process that competes with exciton--exciton annihilation. The interplay between exciton--exciton and exciton--photon annihilation ultimately governs the annihilation dynamics and sets the fundamental limit to the transport efficiency. Our model explains recent experimental results and demonstrates that increased annihilation rates might serve as an explicit fingerprint to differentiate between the response of polaritons and other untargeted effects.

physics.chem-ph↗

arXiv2025

Competing excitonic couplings as origin of mimicked phase transitions in zinc-phthalocyanine single crystals

The optical properties of molecular crystals are largely determined by the excitonic coupling of neighboring molecules. This coupling is extremely sensitive to the arrangement of adjacent molecular units, as their electronic interaction is defined by the relative orientation of the individual transition dipole moments and their wave function overlap. Hence, the optical properties, such as fluorescence, are usually highly anisotropic and good indicators of structural changes during the variation of intensive thermodynamic parameters like temperature or pressure. Here, we discuss the peculiar though archetypical case of $β$-phase zinc-phthalocyanine: In single crystals, we report a sudden change of spectral emission with temperature from a broad, unpolarized Frenkel-exciton type luminescence to a narrow, highly polarized superradiance-like fluorescence below 80 K. Surprisingly, we find that there is no sign of a discrete structural phase transition in this temperature regime. To understand this apparent contradiction, we perform polarization-, temperature- and time-dependent photoluminescence measurements along different crystallographic directions to fully map the emission characteristics of the crystal-exciton. By means of ab-initio calculations on a density functional theory level we conclude that our observations are consistent with a dimer exciton model when considering thermalized electronic states. As such, our study presents a representative case study on a well-established molecular material class demonstrating that caution is advised when attributing discrete changes in electronic observables to a structural phase transition. As we show for zinc-phthalocyanine in its $β$-phase modification, slowly varying excitonic couplings and thermal redistribution of excitations can mimic the same signatures attributed to a structural phase transition.

cond-mat.mtrl-sci↗

arXiv2025

Probing plexciton dynamics with higher-order spectroscopy

Coupling molecular transition dipole moments to surface-plasmon polaritons (SPPs) results in the formation of new optical quasiparticles, i.e., plexcitons. Mixing the specific properties of matter excitations and light modes has proven to be an efficient strategy to alter a variety of molecular processes ranging from chemical reactions to exciton transport. Here, we investigate energy transfer in a plexcitonic system of zinc phthalocyanine (ZnPc) molecules aggregated in the crystalline α-phase and an SPP on a planar gold surface. By tuning the angle of incidence, we vary the degree of mixing between excitonic and SPP character of the excited state. We apply our recently developed higher-order pump-probe spectroscopy to separate the system's fifth-order signal describing the dynamics of two-particle interactions. The time it takes for two quasiparticles to meet and annihilate is a measure of their movement and thus the transport of excitation energy in the system. We find that the transport extracted from the fifth-order signal is surprisingly unaffected by the mixing ratio of exciton and SPP contributions of the plexciton. Using a rate equation model, we explain this behavior by fast transition from the plexcitonic states to many localized excitonic dark states that do not have an SPP contribution. Our results give an indication of how hybrid exciton-plasmon systems should be designed to exploit the delocalization of the involved plasmon modes for improved transport.

physics.chem-ph↗

arXiv2025

DIALECT, a software package for exciton spectra and dynamics in large molecular assemblies from weak to strong light-matter coupling regimes

The software package DIALECT is introduced, which provides the capability of calculating excited-state properties and nonadiabatic dynamics of large molecular systems and can be applied to simulate energy and charge-transfer processes in molecular materials. To this end, we employ the FMO-LC-TDDFTB methodology, which combines the use of the fragment molecular orbital approach with the density-functional tight-binding method and an excitonic Hamiltonian including local and charge-transfer excitations. In this work, we present the features and capabilities of the DIALECT software package in simulating the excited state dynamics of molecules and molecular aggregates using exemplary trajectory surface hopping as well as decoherence corrected Ehrenfest dynamics calculations in the framework of LC-TDDFTB and FMO-LC-TDDFTB. In addition, the capability of simulating the polaritonic excited state properties is highlighted by the calculation of the polariton dispersion of an aggregate of naphthalene molecules. The development of the DIALECT program will facilitate the investigation of exciton and charge transport in large and complex molecular systems, such as biological aggregates, nanomaterials and other complex organic molecular systems.

physics.chem-ph↗

arXiv2024

Anisotropic Photo-Physical Properties of Plexcitons in Strongly Coupled Metal-Organic Thin Films

Exciton plasmon polaritons have gained increasing interests over recent years due to their versatile properties emerging by the underlying light-matter coupling and making them potential candidates for new photonic applications. We have advanced this concept by studying thin films of laterally aligned J-type aggregates of self-assembled tetra-bay phenoxy-dendronized perylene bisimide (PBI) molecules, arranged in a helical manner of three strains on a silver surface. As a result of the interaction between the uniformly aligned dipole moments and the surface plasmons of a thin silver layer underneath, the excitonic state at 1.94 eV evolves into dispersions in absorption and emission, both characterized by a distinct anisotropy. The coupling constant defined by the scalar product of the transition dipole moment $\vecμ$ and the surface plasmon wavevector $\vec{k}_x$ shows a pronounced two-fold rotational symmetry with values between almost 0 to 28 meV. Complementary TD-DFT calculations of the angular dependent absorption and photoluminescence provide insights in the coherent energy exchange between the excitonic and plasmonic sub-systems. Additionally, power dependent PL studies yield first evidence that the diffusion length of the coupled exciton-plasmon polaritons exceeds that of the mere Frenkel state in neat PBI by at least one order of magnitude. Our results not only demonstrate the possibility to control the photo-physical properties of strongly coupled states by their spatially anisotropic light-matter interaction but also reveal innovative strategies to influence opto-electronic device operation by the directional transport of hybrid state energy.

physics.optics↗

arXiv2024

Onset of Spin Entanglement in Doped Carbon Nanotubes Studied by EPR

Nanoscale semiconductors with isolated spin impurities have been touted as promising materials for their potential use at the intersection of quantum, spin, and information technologies. Electron paramagnetic resonance (EPR) studies of spins in semiconducting carbon nanotubes have overwhelmingly focused on spins more strongly localized by $\rm sp^3$-type lattice defects. However, the creation of such impurities is irreversible and requires specific reactions to generate them. Shallow charge impurities, on the other hand, are more readily and widely produced by simple redox chemistry, but have not yet been investigated for their spin properties. Here we use EPR to study p-doped (6,5) semiconducting single-wall carbon nanotubes (s-SWNTs) and elucidate the role of impurity-impurity interactions in conjunction with exchange and correlation effects for the spin behavior of this material. A quantitative comparison of the EPR signals with phenomenological modeling combined with configuration interaction electronic structure calculations of impurity pairs shows that orbital overlap, combined with exchange and correlation effects, causes the EPR signal to disappear due to spin entanglement for doping levels corresponding to impurity spacings of $14\,\rm nm$ (at 30 K). This transition is predicted to shift to higher doping levels with increasing temperature and to lower levels with increasing screening, providing an opportunity for improved spin control in doped s-SWNTs.

cond-mat.mtrl-sci↗

arXiv2024

Prediction of Fluorescence Quantum Yields using the Extended Thawed Gaussian Approximation

Spontaneous emission and internal conversion rates are calculated within harmonic approximations and compared to results obtained within the semi-classical extended thawed Gaussian approximation. This is the first application of the ETGA in the calculation of internal conversion and emission rates for real molecular systems, namely formaldehyde, fluorobenzene, azulene and a dicyano-squaraine dye. The viability of the models as black-box tools for prediction of spontaneous emission and internal conversion rates is assessed. All calculations were done using a consistent protocol in order to investigate how different methods perform without previous experimental knowledge. Contrasting the results with experimental data shows that there are further improvements required before theoretical predictions of emission and internal conversion rates can be used as reliable indicator for the photo-luminescence properties of molecules. We find that the extended thawed Gaussian approximation performs rather similar to the vertical harmonical model. Including anharmonicities in the calculation of internal conversion rates has a moderate effect on the quantitative results in the studied systems. The electronic structure calculations were done using the B3LYP, PBE0, $ω$B97XD and CAM-B3LYP functionals. The choice of the functional does not appear to be a major limiting factor for a black-box approach, when it comes to the prediction of radiative and nonradiative rates for organic molecules. The emission rates are fairly stable with respect to computational parameters, but the internal conversion rate reveals itself to be highly dependent on the choice of the spectral lineshape function, particularly the width of the Lorentzian function, associated with homogeneous broadening.

physics.chem-ph↗

arXiv2022

Internal Conversion Rates from the Extended Thawed Gaussian Approximation: Theory and Validation

The theoretical prediction of the rates of nonradiative processes in molecules is fundamental to assess their emissive properties. In this context, global harmonic models have been widely used to simulate vibronic spectra as well as internal conversion rates and to predict photoluminescence quantum yields. However, these simplified models suffer from the limitations that are inherent to the harmonic approximation and can have a severe effect on the calculated internal conversion rates. Therefore, the development of more accurate semiclassical methods is highly desirable. Here, we introduce a procedure for the calculation of nonradiative rates in the framework of the time-dependent semi-classical Extended Thawed Gaussian Approximation (ETGA). We systematically investigate the performance of the ETGA method by comparing it to the the adiabatic and vertical harmonic method, which belong to the class of widely used global harmonic models. Its performance is tested in potentials that cannot be treated adequately by global harmonic models, beginning with Morse potentials of varying anharmonicity followed by a double well potential. The calculated radiative and nonradiative internal conversion rates are compared to reference values based on exact quantum dynamics. We find that the ETGA has the capability to predict internal conversion rates in anharmonic systems with an appreciable energy gap, whereas the global harmonic models prove to be insufficient.

physics.chem-ph↗

arXiv2022

Long-range Corrected Fragment Molecular Orbital Density-Functional Tight-binding Method for Excited States in Large Molecular Systems

Herein, we present a new method to efficiently calculate electronically excited states in large molecular assemblies, consisting of hundreds of molecules. For this purpose, we combine the long-range corrected tight-binding density-functional fragment molecular orbital method (FMO-LC-DFTB) with an excitonic Hamiltonian, which is constructed in the basis of locally excited and charge-transfer configuration state functions calculated for embedded monomers and dimers and accounts explicitly for the electronic coupling between all types of excitons. We first evaluate both the accuracy and efficiency of our fragmentation approach for molecular dimers and aggregates by comparing with the full LC-TD-DFTB method. The comparison of the calculated spectra of an anthracene cluster shows a very good agreement between our method and the LC-TD-DFTB reference. The effective computational scaling of our method has been explored for anthracene clusters and for perylene bisimide aggregates. We demonstrate the applicability of our method by the calculation of the excited state properties of pentacene crystal models consisting of up to 319 molecules. Furthermore, the participation ratio of the monomer fragments to the excited states is analyzed by the calculation of natural transition orbital (NTO) participation numbers, which are verified by the hole and particle density for a chosen pentacene cluster. The use of our FMO-LC-TDDFTB method will allow for future studies of excitonic dynamics and charge transport to be performed on complex molecular systems consisting of thousands of atoms.

physics.chem-ph↗

arXiv2022

Quantum-classical Dynamics of Vibration-Induced Autoionization in Molecules

We present a novel method for the simulation of the vibration-induced autoionization dynamics in molecular anions in the framework of the quantum-classical surface hopping approach. Classical trajectories starting from quantum initial conditions are propagated on a quantum-mechanical potential energy surface while allowing for autoionization through transitions into discretized continuum states. These transitions are induced by the couplings between the electronic states of the bound anionic system and the electron-detached system composed of the neutral molecule and the free electron. A discretization scheme for the detached system is introduced and a set of formulae is derived which enables the approximate calculation of couplings between the bound and free-electron states. We demonstrate our method on the example of the anion of vinylidene, a high-energy isomer of acetylene, for which detailed experimental data is available. Our results provide information on the time scale of the autoionization process and give an insight into the energetic and angular distribution of the ejected electrons as well as into the associated changes of the molecular geometry. We identify the formation of structures with reduced C-C bond lengths and T-like conformations through bending of the CH$_2$ group with respect to the C-C axis and point out the role of autoionization as a driving process for the isomerization to acetylene.

physics.chem-ph↗

arXiv2021

The Role of Molecular Arrangement on the Dispersion in Strongly Coupled Metal-Organic Hybrid Structures

Metal-organic hybrid structures have been demonstrated a versatile platform to study primary aspects of light-matter interaction by means of emerging states comprising excitonic and plasmonic properties. Here we are studying the wave-vector dependent photo-excitations in gold layers covered by molecular films of zinc-phthalocyanine and its fluorinated derivatives (FnZnPc, with n = 0,4,8,16). These layered metal-organic samples show up to four anti-crossings in their dispersions correlating in energy with the respective degree of ZnPc fluorination. By means of complementary structural and theoretical data, we attribute the observed anti-crossings to three main scenarios of surface plasmon coupling: i) to aggregated $α$-phase regions within the FnZnPc layers at 1.75 eV and 1.85 eV , ii) to a coexisting F16ZnPc $β$-polymorph at 1.51 eV, and iii) to monomers, preferentially located at the metal interface, at 2.15 eV. Whereas energy and splitting of the monomer anti-crossings depend on strength and average tilting of the molecular dipole moments, the aggregate related anti-crossings show a distinct variation with degree of fluorination. These observations can be consistently explained by a change in FnZnPc dipole density induced by an increased lattice spacing due to the larger molecular van der Waals radii upon fluorination. The reported results prove Au/FnZnPc bilayers a model system to demonstrate the high sensitivity of exciton-plasmon coupling on the molecular alignment at microscopic length scales.

physics.optics↗

arXiv2021

Effect of Varying the TD-lc-DFTB Range-Separation Parameter on Charge and Energy Transfer in a Model Pentacene/Buckminsterfullerene Heterojunction

Density-functional tight binding (DFTB) has become a popular form of approximate density-functional theory (DFT) based upon a minimal valence basis set and neglect of all but two center integrals. We report the results of our tests of a recent long-range correction (lc) for time-dependent (TD) lc-DFTB by carrying out TD-lc-DFTB fewest switches surface hopping (FSSH) calculations of energy and charge transfer times using the relatively new DFTBaby program. An advantage of this method is the ability to run enough trajectories to get meaningful ensemble averages. Our interest in the present work is less in determining exact energy and charge transfer rates than in understanding how the results of these calculations vary with the value of the range-separation parameter (Rlc = 1/μ) for a model organic solar cell heterojunction consisting of a van der Waals complex P/F made up of single pentacene (P) molecule together with a single buckminsterfullerene (F) molecule. The default value of Rlc = 3.03 a0 is found to be much too small as neither energy nor charge transfer is observed until Rlc ~ 10 a0. Tests at a single geometry show that best agreement with high-quality ab-initio spectra is obtained in the limit of no lc (i.e., very large Rlc.) A plot of energy and charge transfer rates as a function of Rlc is provided which suggests that a value of Rlc ~ 15 a0 yields the typical literature charge transfer time of about 100 fs. However, energy and charge transfer times become as high as ~ 300 fs for Rlc ~ 25 a0. A closer examination of the charge transfer process P*/F to P+/F- shows that the initial electron transfer is accompanied by a partial delocalization of the P hole onto F which then relocalizes back onto P, consistent with a polaron-like picture in which the nuclei relax to stabilize the resultant redistribution of charges.

physics.chem-ph↗

arXiv2018

Davydov-Type Excitonic Effects on the Absorption Spectra of Parallel-Stacked and Herringbone Aggregates of Pentacene: Time-Dependent Density-Functional Theory and Time-Dependent Density-Functional Tight Binding

Exciton formation leads to J-bands in solid pentacene. Describing these exciton bands represents a challenge for both time-dependent (TD) density-functional theory (DFT) and for its semiempirical analogue, namely for TD density-functional tight binding (DFTB) for three reasons (i) solid pentacene and pentacene aggregates are bound only by van der Waals forces which are notoriously difficult to describe with DFT and DFTB, (ii) the proper description of the long-range coupling between molecules, needed to describe Davydov splitting, is not easy to include in TD-DFT with traditional functionals and in TD-DFTB, and (iii) mixing may occur between local and charge transfer excitons, which may, in turn, require special functionals. We assess how far TD-DFT and TD-DFTB have progressed towards a correct description of this type of exciton by including both a dispersion correction for the ground state and a range-separated hybrid functional for the excited state. Analytic results for parallel-stacked ethylene are derived which go beyond Kasha's exciton model in that we are able to make a clear distinction between charge transfer and energy transfer excitons. This is further confirmed when it is shown that range-separated hybrids have a markedly greater effect on charge-transfer excitons than on energy-transfer excitons in the case of parallel-stacked pentacenes. TD-DFT calculations with the CAM-B3LYP functional and TD-lc-DFT calculations lead to negligeable excitonic corrections for the herringbone crystal structure, possibly because of an overcorrection of charge-transfer effects. In this case, TD-DFT calculations with the B3LYP functional or TD-DFTB calculations parameterized to B3LYP give the best results for excitonic corrections for the herringbone crystal structure as judged from comparison with experimental spectra and with Bethe-Salpeter equation calculations from the literature.

physics.chem-ph↗

arXiv2018

Ultrafast Photodynamics of Glucose

We have investigated the photodynamics of $β$-D-glucose employing our field-induced surface hopping method (FISH), which allows us to simulate the coupled electron-nuclear dynamics, including explicitly nonadiabatic effects and light-induced excitation. Our results reveal that from the initially populated S$_{1}$ and S$_{2}$ states, glucose returns nonradiatively to the ground state within about 200 fs. This takes place mainly via conical intersections (CIs) whose geometries in most cases involve the elongation of a single O-H bond, while in some instances ring-opening due to dissociation of a C-O bond is observed. Experimentally, excitation to a distinct excited electronic state is improbable due to the presence of a dense manifold of states bearing similar oscillator strengths. Our FISH simulations explicitly including a UV laser pulse of 6.43 eV photon energy reveals that after initial excitation the population is almost equally spread over several close-lying electronic states. This is followed by a fast nonradiative decay on the time scale of 100-200 fs, with the final return to the ground state proceeding via the S$_{1}$ state through the same types of CIs as observed in the field-free simulations.

physics.chem-ph↗

arXiv2018

Optimal control of light propagation and exciton transfer in arrays of molecular-like noble-metal clusters

We demonstrate theoretically the possibility of optimal control of light propagation and exciton transfer in arrays constructed of subnanometer sized noble-metal clusters by using phase-shaped laser pulses and analyze the mechanism underlying this process. The theoretical approach for simulation of light propagation in the arrays is based on the numerical solution of the coupled time-dependent Schrödinger equation and the classical electric field propagation in an iterative self-consistent manner. The electronic eigenstates of individual clusters and the dipole couplings are obtained from ab initio TDDFT calculations. The total electric field is propagated along the array by coupling an external excitation electric field with the electric fields produced by all clusters. A genetic algorithm is used to determine optimal pulse shapes which drive the excitation in a desired direction. The described theoretical approach is applied to control the light propagation and exciton transfer dynamics into a T-shaped structure built of seven Ag8 clusters. We demonstrate that a selective switching of light localization is possible in $\sim$5 nm sized cluster arrays which might serve as a building block for plasmonic devices with an ultrafast operation regime

physics.atm-clus↗

arXiv2017

DFTBaby: A software package for non-adiabatic molecular dynamics simulations based on long-range corrected tight-binding TD-DFT(B)

A software package, called DFTBaby, is published, which provides the electronic structure needed for running non-adiabatic molecular dynamics simulations at the level of charge-consistent tight-binding DFT. A long-range correction is incorporated to avoid spurious charge transfer states. Excited state energies, their analytic gradients and scalar non-adiabatic couplings are computed using tight-binding TD-DFT. These quantities are fed into a molecular dynamics code, which integrates Newton's equations of motion for the nuclei together with the electronic Schrödinger equation. Non-adiabatic effects are included by surface hopping. As an example, the program is applied to the optimization of excited states and non-adiabatic dynamics of polyfluorene. The python and Fortran source code is available at http://www.dftbaby.chemie.uni-wuerzburg.de/.

physics.chem-ph↗

arXiv2016

Fully quantum non-adiabatic dynamics in electronic-nuclear coherent state basis

Direct dynamics methods using Gaussian wavepackets have to rely only on local properties, such as gradients and hessians at the center of the wavepacket, so as to be compatible with the usual quantum chemistry methods. Matrix elements of the potential energy surfaces between wavepackets therefore usually have to be approximated. It is shown, that if a modified form of valence bond theory is used instead of the usual MO-based theories, the matrix elements can be obtained exactly. This is so because the molecular Hamiltonian only contains the Coulomb potential, for which matrix elements between different basis functions (consisting of Gaussian nuclear and electronic orbitals) are all well-known. In valence bond theory the self-consistent field calculation can be avoided so that the matrix elements are analytical functions of the nuclear coordinates. A method for simulating non-adiabatic quantum dynamics is sketched, where coherent state trajectories are propagated "on the fly" on adiabatic potential energy surfaces without making approximations to the matrix elements responsible for the coupling between trajectories.

physics.chem-ph↗