The Experts below are selected from a list of 309 Experts worldwide ranked by ideXlab platform
Massimo Testa - One of the best experts on this subject based on the ideXlab platform.
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The stress tensor of a Molecular System: an exercise in statistical mechanics.
Journal of Chemical Physics, 2006Co-Authors: Silvia Morante, Giancarlo Rossi, Massimo TestaAbstract:We prove that conservation of the stress tensor is a consequence of the invariance of the partition function under canonical diffeomorphisms. From this observation a simple and general derivation of the formula which gives the local expression of the stress tensor of a Molecular System in terms of its microscopic degrees of freedom readily follows. The derivation is valid in the canonical as well as the microcanonical ensemble. It works both in the classical and in the quantum mechanical settings and for arbitrary boundary conditions. In particular, if periodic boundary conditions are assigned to the System, the usual minimal-image prescription is naturally born out for mathematical consistency. An interesting outcome of our general analysis is that only in the case of a short-range interaction potential a truly local formula for the stress tensor can exist.
Min Zhou - One of the best experts on this subject based on the ideXlab platform.
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a new look at the atomic level virial stress on continuum Molecular System equivalence
Proceedings of The Royal Society A: Mathematical Physical and Engineering Sciences, 2003Co-Authors: Min ZhouAbstract:The virial stress is the most commonly used definition of stress in discrete particle Systems. This quantity includes two parts. The first part depends on the mass and velocity (or, in some versions, the fluctuation part of the velocity) of atomic particles, reflecting an assertion that mass transfer causes mechanical stress to be applied on stationary spatial surfaces external to an atomic‐particle System. The second part depends on interatomic forces and atomic positions, providing a continuum measure for the internal mechanical interactions between particles. Historic derivations of the virial stress include generalization from the virial theorem of Clausius (1870) for gas pressure and solution of the spatial equation of balance of momentum. The virial stress is stress‐like a measure for momentum change in space. This paper shows that, contrary to the generally accepted view, the virial stress is not a measure for mechanical force between material points and cannot be regarded as a measure for mechanical stress in any sense. The lack of physical significance is both at the individual atom level in a time‐resolved sense and at the System level in a statistical sense. It is demonstrated that the interatomic force term alone is a valid stress measure and can be identified with the Cauchy stress. The proof in this paper consists of two parts. First, for the simple conditions of rigid translation, uniform tension and tension with thermal oscillations, the virial stress yields clearly erroneous interpretations of stress. Second, the conceptual flaw in the generalization from the virial theorem for gas pressure to stress and the confusion over spatial and material equations of balance of momentum in theoretical derivations of the virial stress that led to its erroneous acceptance as the Cauchy stress are pointed out. Interpretation of the virial stress as a measure for mechanical force violates balance of momentum and is inconsistent with the basic definition of stress. The versions of the virial‐stress formula that involve total particle velocity and the thermal fluctuation part of the velocity are demonstrated to be measures of spatial momentum flow relative to, respectively, a fixed reference frame and a moving frame with a velocity equal to the part of particle velocity not included in the virial formula. To further illustrate the irrelevance of mass transfer to the evaluation of stress, an equivalent continuum (EC) for dynamically deforming atomistic particle Systems is defined. The equivalence of the continuum to discrete atomic Systems includes (i) preservation of linear and angular momenta, (ii) conservation of internal, external and inertial work rates, and (iii) conservation of mass. This equivalence allows fields of work‐ and momentum‐preserving Cauchy stress, surface traction, body force and deformation to be determined. The resulting stress field depends only on interatomic forces, providing an independent proof that as a measure for internal material interaction stress is independent of kinetic energy or mass transfer.
Florentino Borondo - One of the best experts on this subject based on the ideXlab platform.
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Analysis of the Full Vibrational Dynamics of the LiNC/LiCN Molecular System
Progress and Challenges in Dynamical Systems, 2013Co-Authors: P. Benitez, Rosa M. Benito, Juan Carlos Losada, Florentino BorondoAbstract:A study of the LiNC/LiCN triatomic molecule vibrational dynamics including all three degrees of freedom (dof) using Frequency Map and Small Alignment Index (SALI) analysis is presented. SALI maps are computed as two-dimensional phase space representations, where its asymptotic values are represented in a color scale. These maps provide full information on the dynamical phase space structure of the System, regardless of its dimensionality. SALI results for the 3dof LiNC/LiCN are compared with 2dof results previously reported for this molecule by our group. A representation of the SALI values on the frequency space, that allows a deeper analysis of the dynamics of the System, is also studied.
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Vibrational dynamics of the floppy LiNC/LiCN Molecular System.
The Journal of chemical physics, 2005Co-Authors: F. J. Arranz, Rosa M. Benito, Florentino BorondoAbstract:Modern spectroscopical techniques allow the efficient experimental investigation of highly excited vibrational states in Molecular Systems. On the theoretical side, powerful computational methods have also been developed for the calculation of the corresponding energy levels and wave functions, and their interpretation. In this paper we use a combination of two such methods, namely, the distribution of zeroes in the Husimi function and energy-level correlation diagrams, to discuss a classification scheme, for the lowest hundred vibrational levels of the LiNC∕LiCN floppy Molecular System, based on their dynamical characteristics.
J G Hou - One of the best experts on this subject based on the ideXlab platform.
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negative differential resistance in a hybrid silicon Molecular System resonance between the intrinsic surface states and the Molecular orbital
ACS Nano, 2012Co-Authors: Weihua Wang, Hui Zhang, Aidi Zhao, Bing Wang, Jinlong Yang, J G HouAbstract:It has been a long-term desire to fabricate hybrid silicon-Molecular devices by taking advantages of organic molecules and the existing silicon-based technology. However, one of the challenging tasks is to design applicable functions on the basis of the intrinsic properties of the molecules, as well as the silicon substrates. Here we demonstrate a silicon-Molecular System that produces negative differential resistance (NDR) by making use of the well-defined intrinsic surface-states of the Si (111)-√3 × √3-Ag (R3-Ag/Si) surface and the Molecular orbital of cobalt(II)–phthalocyanine (CoPc) molecules. From our experimental results obtained using scanning tunneling microscopy/spectroscopy, we find that NDR robustly appears at the Co2+ ion centers of the CoPc molecules, independent of the adsorption configuration of the CoPc molecules and irrespective of doping type and doping concentration of the silicon substrates. Joint with first principle calculations, we conclude that NDR is originated from the resonance...
Gautam Gangopadhyay - One of the best experts on this subject based on the ideXlab platform.
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Laser cooling of vibrational degrees of freedom of a Molecular System.
The Journal of chemical physics, 2005Co-Authors: Sumana Banerjee, Gautam GangopadhyayAbstract:We consider the cooling of vibrational degrees of freedom in a photoinduced excited electronic state of a model Molecular System. For the various parameters of the potential surfaces of the ground and excited electronic states and depending on the excitation frequency of a single-mode laser light, the average energy or average vibrational temperature of the excited state passes through a minimum. The amount of cooling is quantified in terms of the overlap integral between the ground and excited electronic states of the molecule. We have given an approach to calculate the Franck-Condon factor for a multimode displaced-distorted-rotated oscillator surface of the Molecular System. This is subsequently used to study the effect of displacement, distortion, and Duschinsky rotation on the vibrational cooling in the excited state. The absorption spectra and also the average energy or the effective temperature of the excited electronic state are studied for the above model Molecular System. Considering the non-Con...
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Quantum beat in the pump–probe signal of a Molecular System
Journal of Physics B: Atomic Molecular and Optical Physics, 2003Co-Authors: Sumana Banerjee, Gautam GangopadhyayAbstract:In a pump–probe experiment, the probe signal shows a quantum beat structure which is a modulated exponential decay with the delay time. We have theoretically analysed the factors which affect the depth of these modulations, namely the initial preparation condition of the pump state, correlation of the lifetimes of the vibrational states of the first excited state due to coupling with a quasi-continuum, and lastly the curvatures and displacement of the two excited electronic states. The effects of these factors are studied in the multimode case where the interplay of vibrational frequencies is shown to display the modification of the quantum beat structure.
