Session Z01: Electronic Structure and Dynamics

Coherent steering of photosynthetic excitons with redox-dependent vibronic coupling

Photosynthetic pigment-protein complexes can capture and transport solar energy with high efficiency. It has been shown that the Fenna-Matthews-Olson (FMO) complex can quench excitations in oxidizing redox environments via a cysteine-mediated mechanism to prevent the generation of reactive oxygen species. In this study, we use two-dimensional electronic spectroscopy to map the excited state dynamics of wild-type and cysteine-mutated FMO complexes in both oxidizing and reducing conditions. We find that FMO steers excitations toward quenching sites in oxidizing conditions by modulating the vibronic coupling of its chromophores. Specifically, the oxidized cysteines shift the pigment energies to detune the resonant coupling between excitons and a vibrational mode in bacteriochlorophyll-a. This vibronically-enhanced energy transfer correlates with increased quantum beating signatures in the two-dimensional spectra. We assign these beating signals to excited state vibrational coherences that maintain their phase relationship through the energy transfer process. These results show that the vibronic energy transfer process proceeds through a coherent mechanism and that photosynthesis has evolved to exploit the quantum mechanics of vibronic coupling for photoprotection.   

Modeling Excited States with Restricted Determinants in State Averaged Resonating Hartree-Fock

Nonadiabatic molecular dynamics (NAMD) simulations provide atomistic insights into photochemical processes, rendering them essential to the progression of the field. However, a major challenge for NAMD simulations lies in obtaining balanced representations of diverse excited states without sacrificing computational affordability. We present Resonating Hartree-Fock (ResHF) as a promising electronic structure theory candidate for NAMD simulations. We hypothesize that ResHF's nonorthogonal Slater determinant manifold will allow for balanced representation of both conical intersections and charge transfer states, all the while boasting mean field scaling. In our current work, we present state averaged (SA) ResHF excited state wavefunctions that are built out of restricted Hartree-Fock determinants. We then model aniline excited states to showcase the ability of SA-ResHF to avoid charge transfer errors common to state averaged Complete Active Space methods.   

Clock Transitions Guard Against Spin Decoherence in Singlet Fission

Short coherence times present a primary obstacle in quantum computing and sensing applications. In atomic systems, clock transitions (CTs), formed from avoided crossings in an applied Zeeman field, can substantially increase coherence times. We show how CTs can dampen intrinsic and extrinsic sources of quantum noise in molecules. Conical intersections between two periodic potentials form CTs in electron paramagnetic resonance experiments of the spin-polarized singlet fission photoproduct. We report on a pair of CTs for a two-chromophore molecule in terms of the Zeeman field strength, molecular orientation relative to the field, and molecular geometry.   

Quiqbox: a highly customizable basis set generator

Quantum and classical computers are being applied to solve ab initio problems in physics and chemistry. In the NISQ era, solving the "electronic structure problem" has become one of the major benchmarks for identifying the boundary between classical and quantum computational power. Electronic structure in condensed matter physics is often defined on a lattice grid while electronic structure methods in quantum chemistry rely on atom-centered single-particle basis functions. Grid-based methods require a large number of single-particle basis functions to obtain sufficient resolution when expanding the N-body wave function. Typically, fewer atomic orbitals are needed than grid points but the convergence to the continuum limit is less systematic. To investigate the consequences and compromises of the single-particle basis set selection on electronic structure methods, we need more flexibility than is offered in standard solid-state and molecular electronic structure packages. Thus, we have developed an open-source software tool called "Quiqbox" in the Julia programming language that allows for easy construction of highly customized floating basis sets. This package allows for versatile configurations of single-particle basis functions as well as variational optimization based on automatic differentiation of basis set parameters. Quiqbox also provides interfaces for calculating one- and two-body integrals over the custom basis sets using external libraries. Exporting to Molden file format is also supported. Some applications of this software package will be considered in the context of NISQ quantum devices.   

First Principles Electronic Structure Study of Lanthanide-(III) complexes with pyridine-2,6-dicarboxamide on Au(111) substrate

Development of complex metallo-supramolecular has been heavily imbalanced towards transition metal-based systems, and there is a smaller sub-section made up of rare-earth-based assemblies that have been the focal point for some research groups. In particular, europium (III) and lanthanide (III) complexes have attracted attention due to their well-defined luminescence properties, including hypersensitivity to the coordination environment, narrow bandwidth and millisecond luminescence decay times. In this regard, motivated by experiments, we have performed density functional theory (DFT) calculations on [La(pcam)₃]³⁺ deposited on the Au(111) surface to understand its electronic structure and charge transfer analysis between molecule and Au(111) substrate. Obtained DFT energy bandgap is ca. 3.8 eV, in agreement with the value ~3.5 eV obtained from STM experiments. Slight discrepancies in the energy bandgap might be due to the anion trapped with the molecule or might be due to an approximation in DFT. In this presentation, with the aid of different DFT methods, we will discuss the electronic structure and charge transfer analysis for both single and dimer [La(pcam)₃]³⁺ molecules with and without trapped triflate counter anions, [CF₃SO₃]^-1, on an Au(111) substrate based on Bader charge analysis.   

Nonlocal pseudopotentials and time-step errors in diffusion Monte Carlo

We present a version of the T-moves approach for treating nonlocal pseudopotentials in diffusion Monte Carlo, which has much smaller time-step errors than the existing T-moves approaches, while at the same time preserving desirable features such as the upper-bound property for the energy. In addition, we modify the reweighting factor of the projector used in diffusion Monte Carlo to reduce the time-step error. The latter is applicable not only to pseudopotential calculations but also to all-electron calculations.   

Defect process in bismuth germanante: A precursor to band-edge engineering and design of stable scintillators

Materials used as Scintillators suffer degradation while in operation due to defects induced in the process. Overcoming this menace require an extensive understanding of the defect process. Knowledge of the defect formation process is also important in the design of resilient scintillators. Bismuth germanate Bi₄ Ge₃ O₁₂ (BGO) has been extensively studied as a luminescent material that emits light in the visible region upon exposure to ionizing radiation. In this work, we studied using density functional theory, the formation energies of vacancies/intersitials, cation antisites in pristine BGO. Furthermore, to provide insights into factors affecting the stability of doped systems and unravel the origin of the activity of doped BGO, we investigate the structural properties, energetics of pristine and REE (Nd, Pr, Ce and Tm)- doped BGO using first- principles methods. The electronic structure and optical properties of pristine and dopant systems were also studied. We obtained stable site for the dopant and established relationship between size of the dopant and oxygen vacancy formation energy. The doping has minimal effect on structural properties of BGO. However, we found it to impact on O vacancy formation. Our studies of the effect of REE doping shows introduction of states close to the conduction band and provides theoretical insight into how band engineering can be applied to modify properties of scintillators even at very small concentration. Our analysis of the optical properties reveals variations in different regions of photon energy spectra.   

Quantum Drude Oscillator Model: Optimized parametrization for all elements in the Periodic table.

The quantum Drude oscillator (QDO) provides a minimal and robust model of electrical response properties of matter by replacing the electron cloud oscillations on each atom with an effective quantum harmonic oscillator. Three parameters characterize each QDO (mass, frequency, and charge), usually adjusted to reproduce three atomic response properties. Here, we present a more general parametrization of the QDO, valid for all the elements in the periodic table that uses only the static polarizability α₁ and dipole-dipole C₆ dispersion coefficient. As an additional constraint, we impose the relation between the polarizability and vdW radius, as derived in [Fedorov et al., PRL 121, 183401 (2018); Vaccarelli et al., Phys. Rev. Research 3, 033181 (2021)]. By using only α₁ and C₆ as initial observables, the new parametrized QDO model can accurately describe the response properties of all atoms and different small molecules, with straightforward access to higher-order multipole polarizabilities and vdW radii. In addition to providing accurate and efficient representation of response properties of atoms and molecules, our optimized QDO model brings novel insights into scaling behavior and an upper limit on atomic polarizability   

Tang-Toennies Model Revisited: Toward the General van der Waals Potential

Noncovalent van der Waals (vdW) interactions play an essential role in a wide range of systems throughout physics, chemistry, and biology. Noble-gas dimers serve as natural benchmark systems to develop models for vdW interactions. The potential energy curves of noble-gas dimers are well-described by the analytical Tang-Toennies (TT) model, which was recently improved by exploiting the fact that these potential curves are conformal [Sheng et al., PRL 125, 253402 (2020)]. However, this model requires ab initio data to set the values to its many parameters. Here, based on the ab initio data analysis and simple physical models (hydrogen atom, Quantum Drude Oscillator), we derive the scaling law for the potential well depths and use it to reduce the number of parameters in the TT model. Moreover, we derive the empirical relation which allows us to calculate the dispersion coefficients from static atomic polarizabilities with remarkable accuracy for all s- and p-elements in the Periodic Table. Together with the recently established scaling relation between vdW radius and polarizability [Fedorov et al., PRL 121, 183401 (2018)], our results represent an important step toward the self-consistent pairwise vdW potential which can be further generalized onto more complex systems.   

Assessing MP2 frozen natural orbitals for relativistic electronic structure

The O(N⁶) high computational cost is a bottleneck preventing performing Coupled-Cluster (CC) on large systems, particularly when employing 4-component based relativistic Hamiltonians, for which in practice one often uses uncontracted basis set generating large virtual molecular orbital (VMO) spaces.

The canonical Hartree-Fock (HF) orbitals are not the most compact representation for the post-HF method. Alternatively, using natural orbitals is an efficient way to reduce the orbital space while retaining accuracy. We, therefore, implemented the MP2 frozen natural orbital (FNO) method [1] in the Exacorr code [2], with the particularity that our implementation can generate both complex and quaternion FNOs, and also express these on AO basis. It also allows us to obtain CCSD natural orbitals on AO basis, which can be subsequently used in the analysis.

We have investigated the orbital truncation errors for both correlation energy (at CCSD(T) level) and molecular properties (at CCSD level) such as the electric field gradients at the nuclei (EFGs ), dipole and quadrupole moments for hydrogen halides HX (X=F, Cl, Br, I, At, Ts), and parity violation energy shift for the H₂X₂ systems (X= O, S, Se, Te, Po). We find that using FNOs accelerates the convergence of the correlation energy in a roughly uniform manner across the periodic table and that, with VMO spaces truncated to around half of the complete ones, we obtain reliable estimates for both energies and molecular properties in the complete VMO spaces.

 

[1] T. L. Barr, E. R. Davidson, Phys. Rev. A 1970, 1, 644; A. G. Taube, R. J. Bartlett, J. Chem. Phys. 2008, 128, 164101

[2] J. V. Pototschnig, A. Papadopoulos, D. I. Lyakh, M. Repisky, L. Halbert, A. S. P. Gomes, H- J Aa. Jensen, L. Visscher, J. Chem. Theory. Comput. 2021 (in press), arXiv:2013.08473   

Formation Energies of Charged Defects in Transition Metal Dichalcogenides at Finite Temperature and Pressure

    While the formation energy of charged point defects is often used to estimate defect density in a material, the formation energies obtained from density functional theory usually have a 0 Kelvin reference that makes the defect densities far less accurate at finite temperature. The usual solution to this has been to perform finite-temperature density functional theory, but this process is known to be computationally expensive and not worthwhile for many cases. Here we demonstrate an alternative, wherein the 0 Kelvin density functional theory reference is maintained to reduce computational cost. We use the change in chemical potentials of Mo and Se and in molar free energy of MoSe₂ obtained from experimental data and apply the trends in temperature and pressure to the density functional theory reference to obtain defect formation energies for charged defects in freestanding monolayer MoSe₂. We then apply these formation energies to predict equilibrium defect concentrations in MoSe₂ as a function of temperature and pressure.   

Singlet heterofission in tetracene-pentacene thin-film blends

Singlet fission (SF), the photophysical process where an excited singlet state is converted into two triplets on the ultrafast timescales, is nowadays of utmost interest in the scientific community due to the capability to overcome the Shockley-Queisser limit of single junction solar cells. Among the different approaches to control the key parameters of SF, the combination of two different chromophores is a promising way to tune the energy balance and the electronic coupling. In this work we investigate, by means of ultrafast transient absorption spectroscopy, thin film blends of two prototypical SF chromophores: pentacene (PEN) and tetracene (TET). In blends of 5% PEN we observe, for the first time, heterofission of a singlet from PEN to two triplets on one PEN and one TET, after direct excitation of PEN chromophores. For PEN concentrations exceeding 5% there is no evidence of this process, since homofission outcompetes heterofission thanks to its exothermicity. We therefore show that intermolecular heterofission is observable in weakly interacting systems bridging the gap between studies of doped single crystals and strongly coupled systems such as heterodimers.   

Structure, photophysical properties and paramagnetic broadening in (2-Methylbenzimidazolium)MnCl3.2H2O

A new organic-inorganic hybrid compound (C₈H₉N₂)MnCl₃.2H₂O crystallized in monoclinic P2₁/c structure where Mn²⁺ ion is coordinated octahedrally by five Cl^- ions and one H₂O molecule and forms 1D chain by edge-sharing with the neighbouring Manganese octahedra. Upon UV light excitation, it displays intense red emission with an asymmetric broadening around 685 nm can be due to Mn-Mn magnetic coupling. Hitherto, it is believed that the asymmetric broadening at the longer red region can result from Mn-Mn ferromagnetic coupling. But our magnetic measurement confirms the paramagnetic nature of the compound and rules out the possibility of ferromagnetic interaction. The decay curves at different temperatures show biexponential decay nature where one decay component initially increases and later decreases with increasing temperature but the other decay component monotonously increases. So, it can be confirmed that the later component results from Mn-Mn coupled state since at low temperature, enhanced magnetic interaction causes quenching of emission, hence least τ₂ at 15 K. Therefore, our study reveals, not only ferromagnetic but also paramagnetic materials can exhibit longer red emission due to short-range interaction between Mn²⁺ ions and broadens the emission spectra.