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  • 1
    In: The Journal of Chemical Physics, AIP Publishing, Vol. 155, No. 8 ( 2021-08-28)
    Abstract: This article summarizes technical advances contained in the fifth major release of the Q-Chem quantum chemistry program package, covering developments since 2015. A comprehensive library of exchange–correlation functionals, along with a suite of correlated many-body methods, continues to be a hallmark of the Q-Chem software. The many-body methods include novel variants of both coupled-cluster and configuration-interaction approaches along with methods based on the algebraic diagrammatic construction and variational reduced density-matrix methods. Methods highlighted in Q-Chem 5 include a suite of tools for modeling core-level spectroscopy, methods for describing metastable resonances, methods for computing vibronic spectra, the nuclear–electronic orbital method, and several different energy decomposition analysis techniques. High-performance capabilities including multithreaded parallelism and support for calculations on graphics processing units are described. Q-Chem boasts a community of well over 100 active academic developers, and the continuing evolution of the software is supported by an “open teamware” model and an increasingly modular design.
    Type of Medium: Online Resource
    ISSN: 0021-9606 , 1089-7690
    Language: English
    Publisher: AIP Publishing
    Publication Date: 2021
    detail.hit.zdb_id: 3113-6
    detail.hit.zdb_id: 1473050-9
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  • 2
    Online Resource
    Online Resource
    AIP Publishing ; 2004
    In:  Physics of Plasmas Vol. 11, No. 1 ( 2004-01-01), p. 62-70
    In: Physics of Plasmas, AIP Publishing, Vol. 11, No. 1 ( 2004-01-01), p. 62-70
    Abstract: Fast magnetic reconnection of Petschek-type including moving shock waves and discontinuities in a compressible plasma is studied. Magnetic flux tubes of finite size are reconnected by a localized dissipative electric field pulse. This process generates nonlinear perturbations propagating along the initial current surface. The linear wave problem in the outer regions is solved analytically in terms of the reconnection induced sources which move in different directions and with different speeds along the surface. The time-coordinate representation of the solution is given in form of convolution integrals over the reconnection initializing electric field. As an example, reconnection of flux tubes in a sheared magnetic field geometry is analyzed.
    Type of Medium: Online Resource
    ISSN: 1070-664X , 1089-7674
    Language: English
    Publisher: AIP Publishing
    Publication Date: 2004
    detail.hit.zdb_id: 1472746-8
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  • 3
    In: Physics of Plasmas, AIP Publishing, Vol. 21, No. 6 ( 2014-06-01)
    Abstract: In this paper, the impact of momentum and energy conservation of the collision operator in the kinetic description for Resonant Magnetic Perturbations (RMPs) in a tokamak is studied. The particle conserving differential collision operator of Ornstein-Uhlenbeck type is supplemented with integral parts such that energy and momentum are conserved. The application to RMP penetration in a tokamak shows that energy conservation in the electron collision operator is important for the quantitative description of plasma shielding effects at the resonant surface. On the other hand, momentum conservation in the ion collision operator does not significantly change the results.
    Type of Medium: Online Resource
    ISSN: 1070-664X , 1089-7674
    Language: English
    Publisher: AIP Publishing
    Publication Date: 2014
    detail.hit.zdb_id: 1472746-8
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  • 4
    In: Review of Scientific Instruments, AIP Publishing, Vol. 93, No. 7 ( 2022-07-01)
    Abstract: Improvements in x-ray optics critically depend on the measurement of their optical performance. The knowledge of wavefront aberrations, for example, can be used to improve the fabrication of optical elements or to design phase correctors to compensate for these errors. At present, the characterization of such optics is made using intense x-ray sources, such as synchrotrons. However, the limited access to these facilities can substantially slow down the development process. Improvements in the brightness of lab-based x-ray micro-sources in combination with the development of new metrology methods, particularly ptychographic x-ray speckle tracking, enable characterization of x-ray optics in the lab with a precision and sensitivity not possible before. Here, we present a laboratory setup that utilizes a commercially available x-ray source and can be used to characterize different types of x-ray optics. The setup is used in our laboratory on a routine basis to characterize multilayer Laue lenses of high numerical aperture and other optical elements. This typically includes measurements of the wavefront distortions, optimum operating photon energy, and focal length of the lens. To check the sensitivity and accuracy of this laboratory setup, we compared the results to those obtained at the synchrotron and saw no significant difference. To illustrate the feedback of measurements on performance, we demonstrated the correction of the phase errors of a particular multilayer Laue lens using a 3D printed compound refractive phase plate.
    Type of Medium: Online Resource
    ISSN: 0034-6748 , 1089-7623
    Language: English
    Publisher: AIP Publishing
    Publication Date: 2022
    detail.hit.zdb_id: 209865-9
    detail.hit.zdb_id: 1472905-2
    SSG: 11
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  • 5
    Online Resource
    Online Resource
    AIP Publishing ; 1994
    In:  Review of Scientific Instruments Vol. 65, No. 4 ( 1994-04-01), p. 1125-1126
    In: Review of Scientific Instruments, AIP Publishing, Vol. 65, No. 4 ( 1994-04-01), p. 1125-1126
    Abstract: The studies and the experimental work for constructing an electron cyclotron resonance ion source (ECRIS) for the new tritium linear accelerator (TRILAC) at the Institute of Atomic Physics are presented. The absence of consumable parts, the low neutral pressure, and the high ionization efficiency with light elements make ECRIS very attractive for this accelerator. Some special configurations for the microwave system and for the plasma chamber are presented. A suitable modified structure of a power helix-type traveling wave tube (glass envelope with helix) in conjunction with its corresponding lateral rectangular waveguide tapers is presented. Application of an iris-loaded circular waveguide (traveling wave structure of a linear accelerator) as a slow wave structure is also under study. Problems like unstable tuning and arcing, which are encountered in the known structures, could be avoided by using slow wave structures of traveling wave type.
    Type of Medium: Online Resource
    ISSN: 0034-6748 , 1089-7623
    Language: English
    Publisher: AIP Publishing
    Publication Date: 1994
    detail.hit.zdb_id: 209865-9
    detail.hit.zdb_id: 1472905-2
    SSG: 11
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  • 6
    Online Resource
    Online Resource
    AIP Publishing ; 2013
    In:  The Journal of Chemical Physics Vol. 139, No. 24 ( 2013-12-28)
    In: The Journal of Chemical Physics, AIP Publishing, Vol. 139, No. 24 ( 2013-12-28)
    Abstract: Coherent polarization transfer effects in a coupled spin network have been studied over a wide field range. The transfer mechanism is based on exciting zero-quantum coherences between the nuclear spin states by means of non-adiabatic field jump from high to low magnetic field. Subsequent evolution of these coherences enables conversion of spin order in the system, which is monitored after field jump back to high field. Such processes are most efficient when the spin system passes through an avoided level crossing during the field variation. The polarization transfer effects have been demonstrated for N-acetyl histidine, which has five scalar coupled protons; the initial spin order has been prepared by applying RF-pulses at high magnetic field. The observed oscillatory transfer kinetics is taken as a clear indication of a coherent mechanism; level crossing effects have also been demonstrated. The experimental data are in very good agreement with the theoretical model of coherent polarization transfer. The method suggested is also valid for other types of initial polarization in the spin system, most notably, for spin hyperpolarization.
    Type of Medium: Online Resource
    ISSN: 0021-9606 , 1089-7690
    Language: English
    Publisher: AIP Publishing
    Publication Date: 2013
    detail.hit.zdb_id: 3113-6
    detail.hit.zdb_id: 1473050-9
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  • 7
    In: The Journal of Chemical Physics, AIP Publishing, Vol. 141, No. 15 ( 2014-10-21)
    Abstract: Nuclear Magnetic Relaxation Dispersion (NMRD) of protons was studied in the pentapeptide Met-enkephalin and the amino acids, which constitute it. Experiments were run by using high-resolution Nuclear Magnetic Resonance (NMR) in combination with fast field-cycling, thus enabling measuring NMRD curves for all individual protons. As in earlier works, Papers I–III, pronounced effects of intramolecular scalar spin-spin interactions, J-couplings, on spin relaxation were found. Notably, at low fields J-couplings tend to equalize the apparent relaxation rates within networks of coupled protons. In Met-enkephalin, in contrast to the free amino acids, there is a sharp increase in the proton T1-relaxation times at high fields due to the changes in the regime of molecular motion. The experimental data are in good agreement with theory. From modelling the relaxation experiments we were able to determine motional correlation times of different residues in Met-enkephalin with atomic resolution. This allows us to draw conclusions about preferential conformation of the pentapeptide in solution, which is also in agreement with data from two-dimensional NMR experiments (rotating frame Overhauser effect spectroscopy). Altogether, our study demonstrates that high-resolution NMR studies of magnetic field-dependent relaxation allow one to probe molecular mobility in biomolecules with atomic resolution.
    Type of Medium: Online Resource
    ISSN: 0021-9606 , 1089-7690
    Language: English
    Publisher: AIP Publishing
    Publication Date: 2014
    detail.hit.zdb_id: 3113-6
    detail.hit.zdb_id: 1473050-9
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  • 8
    In: The Journal of Chemical Physics, AIP Publishing, Vol. 143, No. 23 ( 2015-12-21)
    Abstract: A method for precise manipulation of non-thermal nuclear spin polarization by switching a RF-field is presented. The method harnesses adiabatic correlation of spin states in the rotating frame. A detailed theory behind the technique is outlined; examples of two-spin and three-spin systems prepared in a non-equilibrium state by Para-Hydrogen Induced Polarization (PHIP) are considered. We demonstrate that the method is suitable for converting the initial multiplet polarization of spins into net polarization: compensation of positive and negative lines in nuclear magnetic resonance spectra, which is detrimental when the spectral resolution is low, is avoided. Such a conversion is performed for real two-spin and three-spin systems polarized by means of PHIP. Potential applications of the presented technique are discussed for manipulating PHIP and its recent modification termed signal amplification by reversible exchange as well as for preparing and observing long-lived spin states.
    Type of Medium: Online Resource
    ISSN: 0021-9606 , 1089-7690
    Language: English
    Publisher: AIP Publishing
    Publication Date: 2015
    detail.hit.zdb_id: 3113-6
    detail.hit.zdb_id: 1473050-9
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  • 9
    In: The Journal of Chemical Physics, AIP Publishing, Vol. 150, No. 9 ( 2019-03-07)
    Abstract: The magnetic field dependence of Chemically Induced Dynamic Nuclear Polarization (CIDNP) in solid-state systems is analyzed theoretically with the aim to explain the puzzling sign change of polarization found at low fields [D. Gräsing et al., Sci. Rep. 7, 12111 (2017)]. We exploit the analysis of polarization in terms of level crossings and level anti-crossings trying to identify the positions of features in the CIDNP field dependence with specific crossings between spin energy levels of the radical pair. Theoretical treatment of solid-state CIDNP reveals a strong orientation dependence of polarization due to the spin dynamics conditioned by anisotropic spin interactions. Specifically, different anisotropic CIDNP mechanisms become active at different magnetic fields and different molecular orientations. Consequently, the field dependence and orientation dependence of polarization need to be analyzed together in order to rationalize experimental observations. By considering both magnetic field and orientation dependence of CIDNP, we are able to explain the previously measured CIDNP field dependence in photosynthetic reaction centers and to obtain a good qualitative agreement between the experimental observations and theoretical results.
    Type of Medium: Online Resource
    ISSN: 0021-9606 , 1089-7690
    Language: English
    Publisher: AIP Publishing
    Publication Date: 2019
    detail.hit.zdb_id: 3113-6
    detail.hit.zdb_id: 1473050-9
    Library Location Call Number Volume/Issue/Year Availability
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  • 10
    Online Resource
    Online Resource
    AIP Publishing ; 1996
    In:  Review of Scientific Instruments Vol. 67, No. 3 ( 1996-03-01), p. 1183-1185
    In: Review of Scientific Instruments, AIP Publishing, Vol. 67, No. 3 ( 1996-03-01), p. 1183-1185
    Abstract: This article presents a 2.45 GHz electron cyclotron resonance ion source which is under construction at the Institute of Atomic Physics Bucharest, Romania. It will be used as an injector into an existing linac for light ions and is a compact version of a similar source, for which we have obtained the magnetic system by superimposing an axial magnetic field generated by two solenoids and a radial magnetic field generated by a permanent magnet hexapole. For the compact version we have employed two rings made of permanent magnets for the axial magnetic field and the same type of hexapole for the radial field. This second version will work on a high voltage platform. The calculation, design, and construction of the two minimum magnetic field structures (B minimum) are presented. These sources have a special microwave antenna, of a helix type, allowing a much smaller diameter plasma chamber than is usually used for the employed microwaves. The microwave system is also presented. The first experiments were made with an Ar plasma.
    Type of Medium: Online Resource
    ISSN: 0034-6748 , 1089-7623
    Language: English
    Publisher: AIP Publishing
    Publication Date: 1996
    detail.hit.zdb_id: 209865-9
    detail.hit.zdb_id: 1472905-2
    SSG: 11
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