Session Z39: Theory and Computation; Surfaces and Interfaces

Averting the infrared catastrophe in the gold standard of quantum chemistry

Coupled-cluster theories can be used to compute ab initio electronic correlation energies of real materials with systematically improvable accuracy. However, the widely-used coupled cluster singles and doubles plus perturbative triples (CCSD(T)) method is only applicable to insulating materials. For zero-gap materials the truncation of the underlying many-body perturbation expansion leads to an infrared catastrophe. Here, we present a novel perturbative triples formalism denoted as (cT) that yields convergent correlation energies in metallic systems. Furthermore, the computed correlation energies for the three dimensional uniform electron gas at metallic densities are in good agreement with quantum Monte Carlo results. At the same time the newly proposed method retains all desirable properties of CCSD(T) such as its accuracy for insulating systems as well as its low computational cost compared to a full inclusion of the triples. This paves the way for ab initio calculations of real metals with chemical accuracy.   

Capturing symmetric correlation for Fermionic systems with a Jastrow factor based on Spectral Neighbor Analysis Potentials

Quantum Monte Carlo methods offer a route to efficiently and accurately account for electron correlation. QMC methods rely on a trial wave function to guide the importance sampling to allow for efficient sampling yielding low statistical variance. A typically utilized form of the trial wave function is the Slater-Jastrow wave function, where the Jastrow factor accounts for symmetric, dynamic correlations between particles. By capturing these correlations, the Jastrow factor reduces the mixed estimator bias, reduces errors introduced by the use of effective core potentials, and increases the efficiency of diffusion Monte Carlo simulations due to the reduction in variance. Motivated by recent works demonstrating the flexibility of machine learned wave functions, we propose a form of the Jastrow factor inspired by machine-learned interatomic potentials (ML-IAP). In the ML-IAP field, the Spectral Neighbor Analysis Potential (SNAP) utilizes a descriptor bispectrum components to capture the local atomic environment. As a Jastrow factor, the SNAP representation is flexible, enables rapid evaluation, and capable of capturing the dynamic instantaneous interactions in a system that manifest as higher-order many-body correlations. We present an initial assessment of the performance of the SNAP Jastrow. SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525   

Enhanced Twist-Averaging Technique for Magnetic Metals: Applications using Quantum Monte Carlo

We propose an improved twist-averaging scheme for quantum Monte Carlo methods that use converged Kohn-Sham or Hartree-Fock orbitals as the reference. This twist-averaging technique is tailored to sample the Brillouin zone of magnetic metals, though it naturally extends to nonmagnetic systems. The proposed scheme aims to reproduce the reference magnetization and achieves charge neutrality by construction, thus avoiding the large energy fluctuations and the post-processing needed to correct the energies. It shows the most robust convergence of total energy and magnetism to the thermodynamic limit when compared against four other twist-averaging schemes. Diffusion Monte Carlo applications are shown on nonmagnetic Al and ferromagnetic α-Fe. The thermodynamic limit cohesive energy of Al shows an excellent agreement with the experimental result.   

Quantum computing and quantum information approach to quantum dynamics in condensed phase

Many important condensed phase processes can be characterized as quantum mechanical particles of a small degrees of freedom interacting with its thermall bath, ranging from charge transfer in solutions and enzymes to exciton migration in semiconducting polymers and molecular aggregates. As the full space simulation is prohibitively expensive, one usually focuses only on the dynamics of the quantum particles, and traces out the bath's degrees of freedom. This leads to an open quantum system approach. When only focusing on the subsystem, the dynamics is in general non-Markovian. Due to this time non-local interaction, the scaling is usually exponential on classical computers with repespect to propagation time. On the other hand, quantum computer simulation of quantum dynamics might offer an advantage. I will present the quantum algorithms we developed in my group that can capture the exact non-Markovian dynamics, and apply it to charge and exciton transfer processes in the condensed phase environment. I will also quantify the degree of non-Markovianity and analysis its effect in transport properties from quantum information perspective.   

evGW ionization potentials at G0W0 cost

The poles of the single-particle Green’s function correspond to the electron addition and removal energies probed in direct and inverse photoemission experiments, and thus provide a direct link between theory and experiment. One way to extract such energies is to use the GW approximation to the self-energy, a popular method in electronic structure theory. Like other nonlinear methods, the GW quasiparticle equations should be solved self-consistently, however, comparatively few fully self-consistent (scGW) solutions have been performed throughout the years due to their high computational cost. Non-self-consistent alternatives, like the one-shot G₀W₀ demand less computational resources but depend on the mean-field orbitals and energies that are used as input. Furthermore, G₀W₀ energies are not good starting points to obtain accurate neutral excitations using the Bethe-Salpeter Equation (BSE) formalism in finite systems. The simplest way to incorporate more physics is to iterate the eigenvalues (evGW) until they remain self-consistent, an effort that comes at a higher computational cost. In this talk, we will show that including an approximation to the derivative discontinuity (DD) of the nearly correct asymptotic potential (NCAP) exchange-correlation functional yields G₀W₀@NCAP-DD quasiparticle energies comparable to the more expensive evGW@GGA energies.   

Nonadiabatic Dynamics in Two-Dimensional Perovskites Assisted by Machine Learning

An exploration of the on-the-fly non-adiabatic couplings (NAC) for nonradiative relaxation and recombination of excited states in 2D Dion-Jacobson (DJ) lead halide perovskites (LHP) is accelerated by a machine learning approach to ab initio molecular dynamics. Molecular dynamics (MD) of nanostructures composed of heavy elements is performed with use of machine learned force-fields (MLFF), as implemented in Vienna Ab initio Simulation Package (VASP). The force field parameterization is established using on-the-fly learning, which continuously builds a force field using ab initio MD (AIMD) data. At each time step of MD simulation, the total energy and forces are predicted based on the MLFF and if the Bayesian error estimate exceeds a threshold an ab initio calculation is performed, which is used to construct a new force field. Model training and evaluation were performed for a range of DJ-LHP models of different thickness and halide composition. The MLFF-MD trajectories were evaluated against AIMD trajectories to assess level of discrepancy and error accumulation. To examine the practical effectiveness of this approach we have used the MLFF-based MD trajectories to compute NAC and excited-state dynamics. At each stage, results based on machine learning are compared to traditional ab initio based electronic dissipative dynamics. We find that MLFF-MD provides comparable results to AIMDs when the MLFF is trained in a NpT ensemble.   

Capturing Electrochemical Signatures of Real Space Twisted Bilayer Graphene Domains

We solve the steady state Poisson-Nernst-Planck equations inside a 3D nanopipette volume to isolate the electrochemical current response at spatial domains of twisted bilayer graphene. We derive kinetic reaction rates from a modified Marcus-Hush-Chidsey theory combining input from a tight binding model that describes the electronic structure of bilayer graphene. Using the local density of states, reaction rates computed at each spatial coordinate provide a real-space picture of the ionic concentration, flux and current for a given nanopipette radius and position. We observe high rates of redox exchange from AA domains, an effect that reduces with diminished flat bands or averages out with larger cross-section area of the nanopipette. The current maxima may occur on either AA or AB center, whichever confines more number of AA domains inside the nanopipette aperture. Based on the intensity of flat bands and the moire unit cell size, current resolution is highest at the magic angle twist for nanopipettes between 2 nm and 5 nm radii. Using steady state voltammograms, we identify an optimal voltage that maximizes current difference between the domains. Our study lays down the framework to electrochemically capture prominent features of the band structure that arise from spatial domains and deformations in 2D flat-band materials.   

Imaging chiral optoelectronic based on STM-CD

The high performance chiral response has been previously developed and covered in the visible spectrum. However, there is currently a lack of clear experimental maps that correlate local electronic structures with chiral properties in such structures. Here, we combine scanning tunneling microscope (STM) and circular dichroism (CD) to detect the electronic orbit and optical properties of O-BODIPY. Our research has provided a strong foundation for understanding the chiral molecular orbital resulting from the selective absorption of light in the helicene molecule. We have developed an appealing method for imaging chiral optical materials in other systems.   

Harnessing Reduced-Symmetry Molecules to Guide Surface Assemblies into Covalent 2D Organic Materials

On-surface synthesis has emerged as a pivotal approach for assembling covalent 2D organic materials from molecular precursors, with long range order and tailored electronic structures. Yet, while molecular self-assembly on surfaces is often highly ordered, subsequent intra-/intermolecular C–C bond formation between molecular precursors leads to disordered on-surface polymerization. This unpredictability stems from a lack of control over molecular site reactivity and from an accompanying  symmetry loss during dehydrogenation and dehydrohalogenation processes.

Here, we introduce a controlled pathway for these processes, achieved by the targeted functionalization of the pyrrole ring of Zinc Tetraphenylporphyrins. This results in a suite of molecules with consistent 2-fold symmetry. We show that selected functional groups and symmetry confinement of dehydrogenation and dehydrohalogenation reactions lead, under certain conditions to a uniform packing exhibiting long-range order, post on-surface intra-molecular reactions, as observed under STM. Comprehensive XPS/UPS characterizations elucidate the electronic structures of these molecular layers, and by-products of C–C bond formation are tracked using TPD.   

Quenching of Phen Green-SK Quantifies Metal Corrosion in Ethanol-based Environments

The corrosive chemical reactions that occur at metal interfaces are not well understood in non-aqueous environments. We demonstrate that the corrosion of carbon steel 1045 and aluminum can be quantified by chelation enhanced quenching of Phen Green-SK (PGSK) in ethanol-based solutions. We first evaluate the dependence of fluorescence intensity of PGSK on iron and aluminum ions concentrations respectively. Subsequently, we apply PGSK to examine the anodic dissolution of metal corrosion. The observed time-dependent chelation enhanced quenching of PGSK quantifies the corrosion rates of two metals over 24-hour immersion in ethanol-based solutions. The PGSK-based quantification of corrosion is compared to scanning electron microscopy and electrochemical techniques, including open circuit potential and Tafel extrapolation. The corrosion rates calculated from chelation enhanced quenching of PGSK and Tafel extrapolation are in agreement, and both indicate a decrease in corrosion rates over 24-hour. Our work shows PGSK can efficiently quantify anodic chemical reactions at metal interfaces, especially in organic solvents or other non-aqueous environments where the application of electrochemical techniques is limited by the poor conductivity of surrounding medium.   

First-principles study of the tritium formation in γ-LiAlO2 pellets and diffusion into Zircaloy-4 getter

Tritium (T) occurs only in trace amounts in the Earth’s environment. To make T in abundance, nuclear reactions are needed. In tritium-producing burnable absorber rods (TPBARs), γ-LiAlO₂ pellets enriched with ⁶Li isotope are used to produce T. When irradiated in a pressurized water reactor, the LiAlO₂ pellets absorb neutrons, simulating the nuclear characteristics of a burnable absorber rod, and produce T through ⁶Li + n à --> T + α. The T reacts with the Ni-coated Zr-base getter where it is captured and leads to formation of metal hydrides. However, accurate analysis of the T transport through the ceramic pellets and the barrier/cladding system is hampered by the lack of fundamental data about T solubility and diffusivity. In this presentation, using first-principles density functional theory, we will elucidate the formation and diffusion pathways of T species (T, OT, T₂, T₂O, CT₄ etc.) in γ-LiAlO₂ depending on the surface structure, vacancy types and different kinds of impurities. We will also demonstrate how these T species diffuse from surface of γ-LiAlO₂ through the Ni layer into the Zircaloy-4 getter to form hydrides. Our results provide a better understanding of T transport properties within TPBARs to improve performance and increase T production with high confidence.   

Unravelling CdSe Nanocrystal Surface Structures with Relativistic DFT Calculations of Solid-State NMR Spectra

Semiconductor nanocrystals (NCs) offer novel tunable electronic and optical properties depending on their size, shape and surface passivation. Solid-state nuclear magnetic resonance (SSNMR) is a powerful tool to determine the structure of NC surfaces. We performed relativistic DFT calculations of cadmium and selenium magnetic shielding tensors with (i) the 4-component Dirac-Kohn-Sham (DKS) Hamiltonian, and (ii) the scalar and (iii) spin-orbit levels within the ZORA Hamiltonian. Molecular clusters with Cd and Se sites in varying bonding environments were used to model CdSe (100) and CdSe (111) nanocrystal surfaces capped with carboxylic acid ligands. Our calculations identify the observed ¹¹³Cd isotropic chemical shifts d(iso) of –465 ppm, –318 ppm and –146 ppm  arising from  CdSeO₃, CdSe₂O₂, and CdSe₃O surface groups respectively, with very good agreement with experiment. The ¹¹³Cd chemical shifts linearly decrease with the number of O-neighbors. The calculations predicted a one-bond ¹¹³Cd-⁷⁷Se scalar coupling of 258 Hz, in good agreement with the experimental values of 250 Hz. Relativistic DFT simulations aid in interpretation of NMR spectra of nanomaterials, and offer new insights into the complex nanocrystal surface structures.   

Oral: Understanding growth dynamics and stability of Hydrothermally synthesized TMDCs QDs

Quantum dots (QDs), due to their non-trivial optical properties, have emerged as one of the most exciting and advanced materials, the major reason being the Quantum confinement effect.  Hydrothermal synthesis offers the challenge of stability leading to the unexpected size non-uniformity among the produced QDs. We aim to discuss, through various morphological and optical observation, the fundamental study of the evolution of hydrothermally synthesized Molybdenum Disulfide QDs considering 'reaction time' as the controlling parameter. We claim that QDs show nucleation, growth, sheet formation (through aggregation) for a smaller reaction time (<14 hours), and sheet fragmentation leading to the formation of smaller-sized QDs at larger reaction times (>18 hours). Fractal Analysis was used to understand the compactness of the QD sheets. At smaller times sheets showed compact behavior and with increased reaction time fractal nature, producing a lower fractal dimension which got saturated at 1.82. Fragmentation events, being the main reason behind the decrease in QD sizes, estimated through fractal analysis of the QD sheets prepared through aggregation of the QDs. Such QDs also shows a higher stability while dispersing in a solvent other than water, hinting towards a long standing problem or size uniformity during QD production by reducing the aggregation events.   

Towards controlling local excitons with chiral edge states in the terraced bilayer 1T'-WTe2/CrI3

The promise of new confinement-driven excitonic and spintronic physics in 2D materials has put them at the forefront of next-generation microelectronics research. In particular, 2D materials like monolayer (ML) CrI3 and WTe2 exhibit interesting magnetooptical and topological properties which can be combined to realize novel multifunctional heterostructures. Here, we present: (1) a new scheme for describing local excitons in ML CrI3 which features an excited-state single-determinant of natural orbitals obtained from iterative selected configuration interaction expansions starting from mean-field single-particle orbitals, and (2) evidence for the possibility of realizing dissipationless chiral edge states in a terraced CrI3/WTe2 bilayer (BL) heterostructure. Importantly, we recover the same local exciton shape in CrI3 as GW-BSE but use only a single determinant of optimized orbitals at a single k-point to do so; we also describe the atomic orbital characteristics of this exciton for the first time. Lastly, we quantify charge transfer, magnetic, and topological effects in BL CrI3/1T'-WTe2, using Density Functional Theory and hybrid Wannier charge centers, finding that CrI3 induces ferromagnetism on W atoms in 1T'-WTe2 and renders the bilayer topologically trivial. This suggests the possibility of realizing chiral edge conductance at the interface of a topological nonmagnetic Weyl semimetal by covering part of it with a 2D ferromagnet.