Bulletin of the American Physical Society
APS March Meeting 2024
Monday–Friday, March 4–8, 2024; Minneapolis & Virtual
Session CC04: V: Computational Physics IIFocus Virtual Only
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Sponsoring Units: DCOMP Chair: Arnold Tharrington, Oak Ridge National Laboratory; Ming Li Room: Virtual Room 04 |
Monday, March 4, 2024 4:00PM - 4:36PM |
CC04.00001: Anomalous Underscreening in Electrolytes and Ionic Liquids Invited Speaker: Andreas Härtel The way electrolytes and ionic liquids screen electric fields is crucial for many applications and studies across several fields. In particular, it is key for capacitive technologies like energy storage and energy conversion devices. During the last years, experiments discovered an unexpected long decay length in concentrated electrolytes that can be orders of magnitude longer than the Debye screening length [1,2]. A variety of theoretical approaches has been applied to explain this finding, including liquid state theories, classical density functional theory, and full-atom molecular dynamics simulations, but they all could not find and explain the anomalous underscreening. However, in a recent study we applied molecular dynamics and Monte Carlo simulations to less concentrated systems and found identical behaviour like the observed underscreening [3]. Remarkably, we could obtain the same results from a direct measurement of the charge correlations as well as from an analysis of clusters of particles. The latter led us to a description of underscreening by a minimal theory of ion pairing that, somehow, even works in concentrated electrolytes. In my presentation I will discuss the discovery and controversial discussion about underscreening, present our findings from simulations and analysis of our data, and demonstrate how a minimal theory can be deduced. Further, I will present strong reasons, why observing anomalous underscreening in dense electrolytes directly from simulations might be more demanding than expected, if not even impossible. |
Monday, March 4, 2024 4:36PM - 4:48PM |
CC04.00002: Investigating Light Emission Efficiency of W(CO)_6 Complexes: Analysis with bpy Ligand Variations KAMRUN NAHAR KEYA, Wenjie Xia, Bakhtiyor Rasulev, Svetlana Kilina, Wenfang Sun, Dmitri Kilin Transition-metal complexes (TMCs) are a pivotal class of compounds with promising applications in optoelectronics, solar energy conversion, and the biomedical domain. In this study, we present photoluminescence data for six W(CO)_6 complexes. We selected six models of these complexes to shed light on the photoluminescence (PL) mechanism inherent to transition-metal complexes. By melding the ab initio electronic structure with a time-dependent density matrix approach, we deciphered the photo-induced, time-variant excited-state dynamics. Our findings emphasized the phonon-induced relaxation of the photoexcited state in W(CO)_6. Utilizing the dissipative Redfield equation of motion, we determined the dissipative excited state dynamics of electronic degrees of freedom, taking nonadiabatic couplings as parameters. Our analyses revealed that the rate of electron relaxation outpaces that of the hole. We further integrated the dissipative excited-state dynamics with radiative recombination channels to predict the PL spectrum. The bidentate ligands we examined consist of a pyridine and an imidazole moiety, conjoined by a C–C bond. Notably, these complexes exhibited heightened phosphorescence from the metal-to-ligand charge transfer (MLCT) excited state, marked by quantum yields. Upon initial photoexcitation, an electron transitioned from an occupied state (represented in blue) to an unoccupied state (depicted in orange). Over time, the populations of the electron/hole states underwent changes, exhibiting an intermediate lifetime and a distinct charge distribution pattern. In essence, our study accentuates the swifter relaxation dynamics of electrons relative to holes during MLCT charge transfers. A pivotal observation from our results is the formation of two bonds of W(CO)_6 spanning an emission range from UV to the infrared spectrum. |
Monday, March 4, 2024 4:48PM - 5:00PM |
CC04.00003: Local field corrections to the retarded cumulant Green's function John J Rehr, Joshua J Kas Local field corrections (LFC) to the screened coulomb potential beyond the RPA have been found to be important in calculations of exchange and correlation properties, particularly at low densities rs=10 and highter. Typically these corrections are included by adding a static momentum dependent Hubbard correction G(q) to the density response function Χ = Χ0 + Χ0 K Χ, where K(q) = vq (1-G(q)) = vq + fxc(q), Χ0(q,ω) is the Lindhard function, and fxc is the TDDFT kernel. The local field factor G(q) = q2 Kxc(q)/4π is parabolic in q at small q and constant near unity at large q, reflecting the crossover from screened to unscreened behavior of the dielectric response with increasing q. We have tested several approximations for G(q) including a simple interpolation formula G(q) =aq2/(1+b q2) with b ~ a = K_xc(0)/4π. Here we report calculations of exchange-correlation properties obtained using the retarded cumulant Green’s function [Phys. Rev. B 90, 085112 (2014), Phys. Rev. B 100, 195144 (2019)] and G0W appraoches with the LFC dielectric function replacing the RPA and compared with accurate quantum Monte Carlo (QMC) results. [Phys. Rep. 744, 1 (2018)]. Although improved accuracy compared with QMC is obtained for the LFC corrected RC approach at rs=10, the agreement becomes significantly worse at rs =20. |
Monday, March 4, 2024 5:00PM - 5:12PM |
CC04.00004: Revisiting Scaling Assumptions of Ab-Initio Calculations Rohit Goswami Density functional theory and ab initio calculations dominate the share of computational physics calculations on high performance compute clusters across the world. A lot of the code driving these calculations (and the related quantum chemistry calculations) are both closed source (e.g., ORCA) and even commercial (e.g., VASP). The scaling of these methods is typically taken to be O(N^3) in time, based on the number of electrons. Of late, machine learning models have been touted as viable alternatives for true ab initio calculations, often using automatic differentiation. We will present the poor scaling of machine learning techniques in space and time, and demonstrate how advances in computational methods and hardware (GPUs, TPUs) have made these "back of the envelope" scaling calculations redundant. Additionally, we present results using complex numbers for generating near-analytic function and gradient values with high accuracy and at a fraction of the computational effort of using finite difference methods (for the same accuracy) while also requiring none of the high-memory usage of automatic differentiation (which stores in memory a complete computational graph for each calculation). It is expected that these results will hopefully prevent further misinformed discussions on the scaling of ab-initio methods and clarify the true scaling of these methods when implemented with modern techniques directed at modern hardware. |
Monday, March 4, 2024 5:12PM - 5:24PM |
CC04.00005: RT-EOM-CCSD Inner and Outer Valence Ionization Energies Fernando D Vila, John J Rehr, Karol Kowalski, Bo Peng Photoelectron spectroscopy (PES), from the UV to X-rays, is a fundamental experimental method for the characterization of materials ranging from adsorbed molecules to catalysts to metals in solution. Despite this broad applicability, the analysis of PES is often hampered by its reliance on the use of standard materials. Therefore, to facilitate the study of unknown systems, advanced theoretical methods for the simulation of PES are highly desirable. In this talk we present the successful deployement of the real-time equation-of-motion coupled cluster singles and doubles (RT-EOM-CCSD) approach[1] to the calculation of the inner and outer valence PES, extending the method's original target of core level spectra to the full spectrum. We show calculations for a series of small molecules which yield ionization energies and total spectral functions that are in good agreement with experiment and of comparable quality (±0.2 eV) to CI-based methods, even for modestly sized basis set. In particular RT-EOM-CCSD provides results that compare well with experiment for inner valence ionizations where correlation is important. |
Monday, March 4, 2024 5:24PM - 5:36PM |
CC04.00006: First principles studies of the electronic and optical properties of two-dimensional arsenic-phosphorous (2D-As-P) compounds Jose Mario Galicia Hernandez, Jonathan Guerrero Sanchez, Rodrigo Ponce Perez, Noboru Takeuchi We modeled 2D hybrid systems based on arsenene and phosphorene, comprising one hexagonal array and three possible orthorhombic arrangements. From phonon spectra we assured the dynamical stability. The computatuon of elastic constants and the cohesive energies ssured the mechanical and thermodynamic stabilities respectively. We also studied the electronic and optical properties. The band structures suggest high mobilities of charge carriers as the ones observed in phosphorene. The computation of band gap was performed by using the GW approximation. For the hexagonal structure, the indirect band gap was 3.63 eV, laying in the limit of visible spectrum and near ultraviolet. Conversely, the two stable orthorhombic structures show direct band gaps of 1.84 and 2.39 eV respectively, inside the visible range, allowing several applications in optoelectronic devices. To study the optical properties, we computed the dielectric function imaginary part within the BSE approach, obtaining exciton binding energies in the range between 550 and 660 meV, favorable to avoid recombination processes. We clalculated the absorption coefficient, which reveals strong absorption in the infrared and visible spectra. From our calculations, it can be established that the 2D-As-P systems are good candidates for several technological applications. |
Monday, March 4, 2024 5:36PM - 5:48PM |
CC04.00007: Impact of Mechanical Deformation on Phonon Hydrodynamics in Two-Dimensional Materials Jamal Abou Haibeh, Samuel Huberman The route to room temperature phonon hydrodynamics remains an open challenge. One unexplored avenue is to engineer phonon properties via mechanical deformation. By leveraging first-principles calculations, this work elucidates the interplay between mechanical strain, compression, and phonon hydrodynamics in 2D (graphene, hexagonal boron nitride) and quasi-2D materials (bi-layer graphene and graphite). First, we investigate the effect of strain on phonon dispersion relations, noting considerable shifts in phonon frequency. Furthermore, by analyzing the phonon dispersion relations in graphite, we observe that a decrease in interlayer distance correspondingly elevates the frequency of the ZO' mode, attributed to the enhanced van der Waals interactions between the layers. Consequently, stronger interlayer interactions induce stiffer layer coupling, manifesting as higher vibrational frequencies. Secondly, we examine the impact of strain on 3rd and 4th order force constants and anharmonic scattering rates. We then examine the impact of tensile and compressive strain on the phonon hydrodynamics window. To do so, we employ Guyer's criteria and solutions to the full scattering matrix Boltzmann transport equation to obtain the window of phonon hydrodynamics in the temperature-length scale phase space as a function of mechanical deformation. We demonstrate that as strain and compression are applied, the hydrodynamic window may shift to higher temperatures. |
Monday, March 4, 2024 5:48PM - 6:00PM |
CC04.00008: First-principles Wigner formulation of coupled radiative and conductive heat transfer Barnabé Ledoux, Mike C Payne, Michele Simoncelli At ordinary temperatures, heat transfer in dielectric solids is mainly mediated by atomic vibrations, as accurately described by established quantum transport formulations for heat conduction. At extremely high temperatures, experiments in polar dielectrics show a very strong enhancement in their heat-transfer capability, which departs from predictions obtained using state-of-the-art conduction theories. Such behavior has been speculated to originate from the emergence of additional radiative effects, but no theoretical framework has been able to rationalize it from first principles. |
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