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关键词： Dielectric properties   Electric polarization   Permittivity  

摘要： The static dielectric constant ϵ0 and its temperature dependence for liquid water is investigated using neural network quantum molecular dynamics (NNQMD). We compute the exact dielectric constant in canonical ensemble from NNQMD trajectories using fluctuations in macroscopic polarization computed from maximally localized Wannier functions (MLWF). Two deep neural networks are constructed. The first, NNQMD, is trained on QMD configurations for liquid water under a variety of temperature and density conditions to learn potential energy surface and forces and then perform molecular dynamics simulations. The second network, NNMLWF, is trained to predict locations of MLWF of individual molecules using the atomic configurations from NNQMD. Training data for both the neural networks is produced using a highly accurate quantum-mechanical method, DFT-SCAN that yields an excellent description of liquid water. We produce 280×106 configurations of water at 7 temperatures using NNQMD and predict MLWF centers using NNMLWF to compute the polarization fluctuations. The length of trajectories needed for a converged value of the dielectric constant at 0°C is found to be 20 ns (40×106 configurations with 0.5 fs time step). The computed dielectric constants for 0, 15, 30, 45, 60, 75, and 90°C are in good agreement with experiments. Our scalable scheme to compute dielectric constants with quantum accuracy is also applicable to other polar molecular liquids.

关键词： 量子力学   研究  

ISBN: (纸本)9787571006778

摘要： 当量子力学的不确定性推翻了牛顿的有序世界时,爱因斯坦和薛定谔正处在这场物理学革命的最前沿。他们向量子力学最荒谬的方面——随机性发起了挑战。本书讲述了爱因斯坦和薛定谔是如何寻求一种能够超越量子的怪异的理论。虽然最终失败了——但读者能够从一个全新的角度一窥物理学的历史,以及两位科学家的生活,他们的执着推动了物理学不断前进。虽然他们试图通过纯粹的几何解释宇宙万物的努力失败了,弦理论的发展,却以量子的方式,让万物理论的想法重新变得流行起来。

关键词： Reaction rates  

摘要： This article presents the application of the reactive step molecular dynamics simulation method [M. Biedermann, D. Diddens, and A. Heuer, J. Chem. Theory Comput. 17, 1074 (2021)] toward two different atomistic, chemically reactive systems. During reactive steps, transitions from reactant to product molecules are modeled according to physically correct transition probabilities based on quantum chemical information about the reactions such as molecular reaction rates via instant exchange of the employed force field and a subsequent, short relaxation of the structure. In the first application, we study the follow-up reactions of singly reduced ethylene carbonate (EC) radicals in EC solution, first, via extensive ab initio molecular dynamics simulations and, second, with the reactive step algorithm. A direct comparison of both simulation methods shows excellent agreement. Then, we employ the reactive step algorithm to simulate the enolate formation of 2-methylcyclopropanone with the base lithium diisopropylamine. Thereby, we can demonstrate that the reactive step algorithm is also capable of capturing effects from kinetic vs thermodynamic control of chemical reactions during simulation.

关键词： Spectral fractional Laplacian   Fourier-like basis function   Spectral method   Error analysis  

摘要： We develop efficient spectral methods for the spectral fractional Laplacian equation and parabolic PDEs with spectral fractional Laplacian on rectangular domains. The key idea is to construct eigenfunctions of discrete Laplacian (also referred to Fourier-like basis) by using the Fourierization method. Under this basis, the non-local fractional Laplacian operator can be trivially evaluated, leading to very efficient algorithms for PDEs involving spectral fractional Laplacian. We provide a rigorous error analysis of the proposed methods for the case with homogeneous boundary conditions, as well as ample numerical results to show their effectiveness.

关键词： Molecular dynamics   Molecular modeling   Cations   Perovskites   Molecules  

摘要： The excited-state quantum dynamics of the organic cation in hybrid perovskites are investigated using the time-dependent density functional theory. The bond fluctuation reveals that the energy relaxation follows different pathways depending on the chemical bonding characteristics within the cation molecule, which can fundamentally affect photostability. For the ammonium-group-containing cations, such as methylammonium (MA) or ethylammonium (EA), local vibrational modes survive for a long time. However, as their lowest unoccupied molecular orbitals (LUMOs) have pi* characters, the amidinium-group-containing cations, such as formamidinium (FA) or guanidinium (GA), efficiently dissipate deposited energy via chaotic intramolecular vibrational energy redistribution. The distinct A-site molecules' dynamics are closely related to the quantum ergodicity, which can bring enhanced photostability of FA and GA compared to MA and EA. Our theoretical investigation reveals the quantum chaos origin of better light stability of FA-based perovskites and serves as the future research direction of the A-site engineering for better solar cells and light-emitting devices.

关键词： Reaction probability   integral cross section   differential cross section   isotope effect  

摘要： The dynamics studies of the Au+ + HD (v (0) = 0, j (0) = 0) -> AuH+/AuD+ + D/H reaction are carried out using time-dependent quantum wave packet and quasi-classical trajectory (QCT) methods. Some meaningful dynamics properties are reported at state-to-state level of theory, such as reaction probability, integral cross section (ICS), and differential cross section (DCS). Furthermore, the ICSs of the two channels of the Au+ + HD reaction are compared with experimental data. Large discrepancies between theory and experiment are observed near the threshold, which can be attributed to the pseudopotential basis set used in the construction of potential energy surface. However, present QCT values are in general good agreement with experimental results as the collision energy above 4.0 eV. The DCSs results indicated that the insert complex-forming reaction mechanism dominates the reaction for these two channels.

关键词： Signless Laplacian spectral radius   k-connected graphs   Signless Laplacian matrix   Diameter  

摘要： Let G(n,k)(D), be the set of all k-connected graphs of order n with diameter D. In this paper, we determine the graphs with maximal signless Laplacian spectral radius among all graphs in G(n,k)(D). (C) 2021 Elsevier Inc. All rights reserved.

关键词： Dynamics  

摘要： Spurred by recent technological advances, there is a growing demand for computational methods that can accurately predict the dynamics of correlated electrons. Such methods can provide much-needed theoretical insights into the electron dynamics probed via time-resolved spectroscopy experiments and observed in non-equilibrium ultracold atom experiments. In this article, we develop and benchmark a numerically exact Auxiliary Field Quantum Monte Carlo (AFQMC) method for modeling the dynamics of correlated electrons in real time. AFQMC has become a powerful method for predicting the ground state and finite temperature properties of strongly correlated systems mostly by employing constraints to control the sign problem. Our initial goal in this work is to determine how well AFQMC generalizes to real-time electron dynamics problems without constraints. By modeling the repulsive Hubbard model on different lattices and with differing initial electronic configurations, we show that real-time AFQMC is capable of accurately capturing long-lived electronic coherences beyond the reach of mean field techniques. While the times to which we can meaningfully model decrease with increasing correlation strength and system size as a result of the exponential growth of the dynamical phase problem, we show that our technique can model the short-time behavior of strongly correlated systems to very high accuracy. Crucially, we find that importance sampling, combined with a novel adaptive active space sampling technique, can substantially lengthen the times to which we can simulate. These results establish real-time AFQMC as a viable technique for modeling the dynamics of correlated electron systems and serve as a basis for future sampling advances that will further mitigate the dynamical phase problem.

关键词： Heusler compounds   Spin metastable state   Zero-field cooling exchange bias  

摘要： The existence of the spin metastable state in magnetic materials often causes novel physics including magnetization jumps, metamagnetic transition, and skyrmions. In this paper, we design the occupancy of Mn atoms in the MnNiGa system, and based on first-principles calculations, we anticipate the existence of spin metastable state in Mn₂Ni_1.5Ga_0.5. Therefore, Mn₂Ni_2−xGa_x (x=0.36-0.60) is selected as the research platform to carry out related research. The experimental results show that near the composition of Mn₂Ni_1.5Ga_0.5, the lattice constant, crystal axis ratio, DC and AC magnetic test all have abnormal changes, and the zero-field cooling exchange bias is observed, which indicates the dual effects of the spin metastable state on structure and magnetic structure, and proves the dominant effect of spin metastable state on zero-field cooling exchange bias. This work provides a new idea for the design of spin metastable states in magnetic materials, and a new method for the realization of a zero-field cold exchange bias system. © 2021.

关键词： Photodynamic therapy   Fluorescence imaging  

摘要： There is a great deal of interest in the development of nanoparticles for biomedicine. The question of how many nanoparticles are taken up by cells is important for biomedical applications. Here, we describe a fluorescence method for the quantitative measurement of the cellular uptake of polymer dots (Pdots) and a further estimation of intracellular Pdots photosensitizer for fluorescence imaging and photodynamic therapy. The approach relies on the high brightness, excellent stability, minimal aggregation quenching, and metalloporphyrin doping properties of the Pdots. We correlated the single-cell fluorescence brightness obtained from fluorescence spectrometry, confocal microscopy, and flow cytometry with the number of endocytosed Pdots, which was validated by inductively coupled plasma mass spectrometry. Our results indicated that, on average, similar to 1.3 million Pdots were taken up by single cells that were incubated for 4 h with arginine 8-Pdots (40 mu g/mL, similar to 20 nm diameter). The absolute number of endocytosed Pdots of individual cells could be estimated from confocal microscopy by comparing the single-cell brightness with the average intensity. Furthermore, we investigated the cell viability as a result of an intracellular Pdots photosensitizer, from which the half maximal inhibitory concentration was determined to be similar to 7.2 x 10(5) Pdots per cell under the light dose of 60 J/cm(2). This study provides an effective method for quantifying endocytosed Pdots, which can be extended to investigate the cellular uptake of various conjugated polymer carriers in biomedicine.