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关键词： Quantum dot   Magnetic properties   Thermodynamic properties   Impurity  

摘要： The effects of magnetic field, temperature, potential parameters and impurity on the magnetic and thermodynamic properties of GaAs quantum dot confined by the parabolic-inverse square plus Gaussian potential are investigated. The Hamiltonian of the system is solved within the effective-mass approach and energy levels are obtained. The results show that temperature, magnetic field, potential parameters and impurity have a great influence on the magnetic and thermodynamic properties of the structure.

关键词： Time dependant density functional theory   Energy   Mathematical methods   Excited states   Potential energy  

摘要： In this contribution we present a quantum dynamical study of the photoexcited hydrogen bonded base pair adenine-thymine (AT) in a Watson-Crick arrangement. To that end, we parametrize Linear Vibronic Coupling (LVC) models with Time-Dependent Density Functional Theory (TD-DFT) calculations, exploiting a fragment diabatization scheme (FrD) we have developed to define diabatic states on the basis of individual chromophores in a multichromophoric system. Wavepacket propagations were run with the multilayer extension of the Multiconfiguration Time-Dependent Hartree method. We considered excitations to the three lowest bright states, a pi pi* state of thymine and two pi pi* states (L-a and L-b) of adenine, and we found that on the 100 fs time scale the main decay pathways involve intramonomer population transfers toward n pi* states of the same nucleobase. In AT this transfer is less effective than in the isolated nucleobases, because hydrogen bonding destabilizes the n pi* states. The population transfer to the A -> T charge transfer state is negligible, making the ultrafast (femtosecond) decay through the proton coupled electron transfer mechanism unlikely, in line with experimental results in apolar solvents. The excitation energy transfer is also very small. We carefully compare the predictions of LVC Hamiltonians obtained with different sets of diabatic states, defined so to match either local states of the two separated monomers or the base pair adiabatic states in the Franck-Condon region. To that end we also extend the flexibility of the FrD-LVC approach, introducing a new strategy to define fragments diabatic states that account for the effect of the rest of the multichromohoric system through a Molecular Mechanics potential.

关键词： Dynamics  

摘要： The transition state is a central concept of chemistry. Photoelectron-photofragment coincidence (PPC) spectroscopy has been proven as an attractive method to study the transition state dynamics. Within a state-of-the-art full-dimensional quantum mechanical model, the dissociative photodetachment dynamics of H3O2- is investigated on accurate anion and neutral potential energy surfaces. The calculated PPC spectrum of H3O2- agrees well with the experimental measurement. The dissociative product OH is exclusively populated on the ground vibrational state, implying the character of the spectator bond. In contrast, the product H2O is predominantly populated in the ground and fundamental states of the symmetric and antisymmetric stretching modes, which is caused by the strong coupling between the antisymmetric motion of the transferred H atom in the transient intermediate [H3O2]* and both stretching modes of the product H2O.

关键词： Molecular dynamics   Chemical calculations   Electronic structure   Computational chemistry   Electrical properties  

摘要： Tensor cores, along with tensor processing units, represent a new form of hardware acceleration specifically designed for deep neural network calculations in artificial intelligence applications. Tensor cores provide extraordinary computational speed and energy efficiency but with the caveat that they were designed for tensor contractions (matrix-matrix multiplications) using only low-precision floating-point operations. Despite this perceived limitation, we demonstrate how tensor cores can be applied with high efficiency to the challenging and numerically sensitive problem of quantum-based Born-Oppenheimer molecular dynamics, which requires highly accurate electronic structure optimizations and conservative force evaluations. The interatomic forces are calculated on-the-fly from an electronic structure that is obtained from a generalized deep neural network, where the computational structure naturally takes advantage of the exceptional processing power of the tensor cores and allows for high performance in excess of 100 Tflops on a single Nvidia A100 GPU. Stable molecular dynamics trajectories are generated using the framework of extended Lagrangian Born-Oppenheimer molecular dynamics, which combines computational efficiency with long-term stability, even when using approximate charge relaxations and force evaluations that are limited in accuracy by the numerically noisy conditions caused by the low-precision tensor core floating-point operations. A canonical ensemble simulation scheme is also presented, where the additional numerical noise in the calculated forces is absorbed into a Langevin-like dynamics.

关键词： 量子测量   哥本哈根解释   “多世界”解释   退相干   多时空  

摘要： 探讨了量子力学诠释中的哥本哈根解释以及“多世界”量子诠释，发现它们间可以有一种折中方案，即“多时空”方案，该方案认为，宇宙中时空可能并不是唯一的，任何系统间的相互作用都会形成一个独立的时空。这些系统在该时空中表现为“粒子”属性，而在其它无关的时空“看来”，这些系统则表现为“波动”属性。在“多时空”方案中，观察者的角色不必如哥本哈根解释中至关重要，而是被沦为了普通地位。同时论证了“多时空”方案下测量的客观性问题。“多时空”方案给量子力学中的测量问题提供了一个新的理解，但是它并未与当前的物理试验有所冲突，仍然居于量子力学的框架之内。

摘要： This thesis is focused on a detailed investigation of the optically in- duced quench switching effect in different films of antiferromagnetic CuMnAs. The quench switching effect was recently discovered to be highly reproducible resistance switching, which can be excited by electrical and optical laser pulses. This thesis compares the amplitude response to laser-induced quench switching for samples on the different substrate material, samples with different stoichiome- tries, and samples with different thicknesses of CuMnAs film. The effects of different ratios between the laser spot and the size of the measured device are in- vestigated, and position-dependent measurements are also presented. It is shown that resistivity change with optical excitation using a single 120 femtosecond laser pulse can, in ideal conditions, reach up to 15% at room temperature, which is comparable with the maximum signal obtained with electrical pulses. All of the measurements combined with current knowledge of quench switching illustrate the robust behavior of this mechanism across a wide range of conditions.

关键词： Free energy   Molecular modeling   Reaction mechanisms   Molecules   Chemical calculations  

摘要： Bacteriorhodopsin (BR) transports a proton from intracellular to extracellular (EC) sites through five proton transfers. The second proton transfer is the release of an excess proton stored in BR into the EC medium, and an atomistic understanding of this whole process has remained unexplored due to its ubiquitous environment. Here, fully quantum mechanical (QM) molecular dynamics (MD) and metadynamics (MTD) simulations for this process were performed at the divide-and-conquer density-functional tight-binding level using realistic models (similar to 50000 and similar to 20000 atoms) based on the time-resolved photointermediate structures from an X-ray free electron laser. Regarding the proton storage process, the QM-MD/MTD simulations confirmed the Glu-shared mechanism, in which an excess proton is stored between Glu194 and Glu204, and clarified that the activation occurs by localizing the proton at Glu204 in the photocycle. Furthermore, the QM-MD/MTD simulations elucidated a release pathway from Glu204 through Ser193 to the EC water molecules and clarified that the proton release starts at similar to 250 mu s. In the ubiquitous proton diffusion in the EC medium, the transient proton receptors predicted experimentally were assigned to carboxylates in Glu9 and Glu74. Large-scale QM-MD/MTD simulations beyond the conventional sizes, which provided the above findings and confirmations, were possible by adopting our DCDFTBMD program.

关键词： Mapping  

摘要： We consider the application of the original Meyer-Miller (MM) Hamiltonian to mapping fermionic quantum dynamics to classical equations of motion. Non-interacting fermionic and bosonic systems share the same one-body density dynamics when evolving from the same initial many-body state. The MM classical mapping is exact for non-interacting bosons, and therefore, it yields the exact time-dependent one-body density for non-interacting fermions as well. Starting from this observation, the MM mapping is compared to different mappings specific for fermionic systems, namely, the spin mapping with and without including a Jordan-Wigner transformation and the Li-Miller mapping (LMM). For non-interacting systems, the inclusion of fermionic anti-symmetry through the Jordan-Wigner transform does not lead to any improvement in the performance of the mappings, and instead, it worsens the classical description. For an interacting impurity model and for models of excitonic energy transfer, the MM and LMM mappings perform similarly, and in some cases, the former outperforms the latter when compared to a full quantum description. The classical mappings are able to capture interference effects, both constructive and destructive, that originate from equivalent energy transfer pathways in the models.

关键词： Charge transfer  

摘要： We present a unified and highly numerically efficient formalism for the simulation of quantum dynamics of complex molecular systems, which takes into account both temperature effects and static disorder. The methodology is based on the thermo-field dynamics formalism, and Gaussian static disorder is included into simulations via auxiliary bosonic operators. This approach, combined with the tensor-train/matrix-product state representation of the thermalized stochastic wave function, is applied to study the effect of dynamic and static disorders in charge-transfer processes in model organic semiconductor chains employing the Su-Schrieffer-Heeger (Holstein-Peierls) model Hamiltonian. Published under an exclusive license by AIP Publishing

关键词： Deep autoencoder   Graph regularization   Sparsity   Dynamic functional connectivity   Brain development  

摘要： Deep-layer autoencoder (DAE) provides a powerful way for medical image analysis, while it remains a daunting challenge due to the limited samples but high dimension. In this paper, a DAE with sparse and graph Laplacian regularization, termed as GSDAE, is presented to identify significant differences of dynamic functional connectivity (dFC) between child and young adult groups. The proposed model incor-porates prior knowledge into sparse learning, i.e., the intrinsic structural information defined by manifold in the data. In this way, the reconstruction ability of unsupervised DAE can be improved, which facilitates the extraction of most discriminative features of dFC changing with age. Results on the fMRI data from the Philadelphia Neurodevelopmental Cohort project reveal essential differences lying in the reoccur-rence patterns of dFC and in the connectivity of resting state networks with increasing age, e.g., there exist different trajectories of connectivity patterns in brain functions: those associated with complex cog-nitive functions generally decreased, while those associated with basic visual or motor control functions usually enhanced. In addition, the brain circuitry moves from segregation to integration during brain development. (c) 2021 Elsevier B.V. All rights reserved.