Browsing by Author "Tang, Ping-Han"

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Analysis of local bond-orientational order for liquid gallium at ambient pressure: Two types of cluster structures
(2016-07-14)
Chen, Lin-Yuan
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Tang, Ping-Han
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Wu, Ten-Ming
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物理研究所
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Institute of Physics
In terms of the local bond-orientational order ( LBOO) parameters, a cluster approach to analyze local structures of simple liquids was developed. In this approach, a cluster is defined as a combination of neighboring seeds having at least n(b) local-orientational bonds and their nearest neighbors, and a cluster ensemble is a collection of clusters with a specified n(b) and number of seeds n(s). This cluster analysis was applied to investigate the microscopic structures of liquid Ga at ambient pressure ( AP). The liquid structures studied were generated through ab initio molecular dynamics simulations. By scrutinizing the static structure factors ( SSFs) of cluster ensembles with different combinations of n(b) and n(s), we found that liquid Ga at AP contained two types of cluster structures, one characterized by sixfold orientational symmetry and the other showing fourfold orientational symmetry. The SSFs of cluster structures with sixfold orientational symmetry were akin to the SSF of a hard-sphere fluid. On the contrary, the SSFs of cluster structures showing fourfold orientational symmetry behaved similarly as the anomalous SSF of liquid Ga at AP, which is well known for exhibiting a high-q shoulder. The local structures of a highly LBOO cluster whose SSF displayed a high-q shoulder were found to be more similar to the structure of beta-Ga than those of other solid phases of Ga. More generally, the cluster structures showing fourfold orientational symmetry have an inclination to resemble more to beta-Ga. Published by AIP Publishing.
Comparative study of cluster Ag(17)Cu(2) by instantaneous normal mode analysis and by isothermal Brownian-type molecular dynamics simulation
(2011-09-07)
Tang, Ping-Han
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Wu, Ten-Ming
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Yen, Tsung-Wen
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Lai, S. K.
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Hsu, P. J.
We perform isothermal Brownian-type molecular dynamics simulations to obtain the velocity auto-correlation function and its time Fourier-transformed power spectral density for the metallic cluster Ag(17)Cu(2). The temperature dependences of these dynamical quantities from T = 0 to 1500 K were examined and across this temperature range the cluster melting temperature T(m), which we define to be the principal maximum position of the specific heat is determined. The instantaneous normal mode analysis is then used to dissect the cluster dynamics by calculating the vibrational instantaneous normal mode density of states and hence its frequency integrated value I(j) which is an ensemble average of all vibrational projection operators for the jth atom in the cluster. In addition to comparing the results with simulation data, we look more closely at the entities I(j) of all atoms using the point group symmetry and diagnose their temperature variations. We find that I(j) exhibit features that may be used to deduce T(m), which turns out to agree very well with those inferred from the power spectral density and specific heat. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3628669]
Comparative study of cluster Ag17Cu2 by instantaneous normal mode analysis and by isothermal Brownian-type molecular dynamics simulation
(2011-09-07)
Tang, Ping-Han
;
Wu, Ten-Ming
;
Yen, Tsung-Wen
;
Lai, S. K.
;
Hsu, P. J.
We perform isothermal Brownian-type molecular dynamics simulations to obtain the velocity auto-correlation function and its time Fourier-transformed power spectral density for the metallic cluster Ag17Cu2. The temperature dependences of these dynamical quantities from T = 0 to 1500 K were examined and across this temperature range the cluster melting temperature T-m, which we define to be the principal maximum position of the specific heat is determined. The instantaneous normal mode analysis is then used to dissect the cluster dynamics by calculating the vibrational instantaneous normal mode density of states and hence its frequency integrated value I-j which is an ensemble average of all vibrational projection operators for the jth atom in the cluster. In addition to comparing the results with simulation data, we look more closely at the entities I-j of all atoms using the point group symmetry and diagnose their temperature variations. We find that I-j exhibit features that may be used to deduce T-m, which turns out to agree very well with those inferred from the power spectral density and specific heat. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3628669]
Instantaneous normal mode analysis for intermolecular and intramolecular vibrations of water from atomic point of view
(2013-11-28)
Chen, Yu-Chun
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Tang, Ping-Han
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Wu, Ten-Ming
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物理研究所
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Institute of Physics
By exploiting the instantaneous normal mode (INM) analysis for models of flexible molecules, we investigate intermolecular and intramolecular vibrations of water from the atomic point of view. With two flexible SPC/E models, our investigations include three aspects about their INM spectra, which are separated into the unstable, intermolecular, bending, and stretching bands. First, the O- and H-atom contributions in the four INM bands are calculated and their stable INM spectra are compared with the power spectra of the atomic velocity autocorrelation functions. The unstable and intermolecular bands of the flexible models are also compared with those of the SPC/E model of rigid molecules. Second, we formulate the inverse participation ratio (IPR) of the INMs, respectively, for the O- and H-atom and molecule. With the IPRs, the numbers of the three species participated in the INMs are estimated so that the localization characters of the INMs in each band are studied. Further, by the ratio of the IPR of the H atom to that of the O atom, we explore the number of involved OH bond per molecule participated in the INMs. Third, by classifying simulated molecules into subensembles according to the geometry of their local environments or their H-bond configurations, we examine the local-structure effects on the bending and stretching INM bands. All of our results are verified to be insensible to the definition of H-bond. Our conclusions about the intermolecular and intramolecular vibrations in water are given. (C) 2013 AIP Publishing LLC.
Instantaneous normal mode analysis for OKE reduced spectra of liquid and supercooled water: Contributions of low-density and high-density liquids
(2020-03-01)
Tang, Ping-Han
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Wu, Ten-Ming
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物理研究所
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Institute of Physics
In this paper, we investigated the optical Kerr effect (OKE) reduced spectral density (RSD) of liquid and supercooled water at ambient pressure and focused on the contributions of low-density liquid (LDL) and highdensity liquid (HDL), which local structures are characterized with tetrahedral order and disorder, respectively. The water systems were generated by performing molecular dynamics (MD) simulations with TIP4P/2005 force fields, and molecules were classified into LDL and HDL with the microscopic structural descriptor [Nature commun. 3 (2014) 35561. The OKE RSD was calculated with a model involving the intrinsic polarizabilities of individual molecules and the dipole-induced-dipole mechanism between them. The instantaneous normal mode (INM) analysis was used to dissect the OKE RSD obtained by MD simulations. By considering water as a mixture of LDL and HDL, the OKE RSD in the linearized INM theory is a sum of three contributions, which are resulted from the two partial liquids individually and a cross correlation associated with both of them, in a formulism similar as that for the static structure factor of a binary mixture. By exploring with two separation schemes on the collective polarizability, where they are complemental to each other, the INM analysis provides an insight into the physical causes for the bands in the OKE RSD of liquid and supercooled water and the relation of each band to the local structures of LDL and HDL (C) 2020 Elsevier B.V. All rights reserved.
Local structural effects on orientational relaxation of OH-bond in liquid water over short to intermediate timescales
(2014-12-07)
Lin, S. R.
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Tang, Ping-Han
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Wu, Ten-Ming
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物理研究所
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Institute of Physics
By simulating the rigid simple point charge extended model at temperature T = 300 K, the orientational relaxation of the OH-bond in water was investigated over short to intermediate timescales, within which molecules undergo inertial rotation and libration and then enter the rotational diffusion regime. According to the second-cumulant approximation, the orientational time correlation function (TCF) of each axis that is parallel or perpendicular to an OH-bond is related to an effective rotational density of states (DOS), which is determined using the power spectra of angular velocity autocorrelation functions (AVAFs) of the other two axes. In addition, the AVAF power spectrum of an axis was approximated as the rotational stable instantaneous normal mode (INM) spectrum of the axis. As described in a previous study [S. L. Chang, T. M. Wu, and C. Y. Mou, J. Chem. Phys. 121, 3605 (2004)], simulated molecules were classified into subensembles, according to either the local structures or the H-bond configurations of the molecules. For global molecules and the classified subensembles, the simulation results for the first-and second-rank orientational TCFs were compared with the second-cumulant predictions obtained using the effective rotational DOSs and the rotational stable-INM spectra. On short timescales, the OH-bond in water behaves similar to an inertial rotor and its anisotropy is lower than that of a water molecule. For molecules with three or more H-bonds, the OH-bond orientational TCFs are characterized by a recurrence, which is an indication for libration of the OH-bond. The recurrence can generally be described by the second-cumulant prediction obtained using the rotational stable-INM spectra; however, the orientational TCFs after the recurrence switch to a behavior similar to that predicted using the AVAF power spectra. By contrast, the OH-bond orientational TCFs of molecules initially connected with one or two H-bonds decay monotonically or exhibit a weak recurrence, indicating rapid relaxation into the rotational diffusion regime after the initial Gaussian decay. In addition to accurately describing the Gaussian decay, the second-cumulant predictions formulated using the rotational stable-INM spectra and the AVAF power spectra serve as the upper and lower limits, respectively, for the OH-bond orientational TCFs of these molecules after the Gaussian decay. (C) 2014 AIP Publishing LLC.
Melting behavior of Ag-14 cluster: An order parameter by instantaneous normal modes
(2012-12-28)
Tang, Ping-Han
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Wu, Ten-Ming
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Hsu, P. J.
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Lai, S. K.
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物理研究所
This paper studies the melting behavior of Ag-14 cluster employing the instantaneous normal mode (INM) analysis that was previously developed for bimetallic cluster Ag17Cu2. The isothermal Brownian-type molecular dynamics simulation is used to generate atom configurations of Ag-14 at different temperatures up to 1500 K. At each temperature, these atomic configurations are then analyzed by the INM technique. To delve into the melting behavior of Ag-14 cluster which differs from Ag17Cu2 by the occurrence of an anomalous prepeak in the specific heat curve in addition to the typical principal peak, we appeal to examining the order parameter tau(T) defined in the context of the INM method. Two general approaches are proposed to calculate tau(T). In one, tau(T) is defined in terms of the INM vibrational density of states; in another, tau(T) is defined considering the cluster as a rigid body with its rotational motions described by three orthogonal eigenvectors. Our results for Ag-14 by these two methods indicate the mutual agreement of tau(T) calculated and also the consistent interpretation of the melting behavior with the specific heat data. The order parameter tau(T) provides in addition an insightful interpretation between the melting of clusters and the concept of broken symmetry which has been found successful in studies of the melting transition of bulk systems. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4772096]
Molecular dynamics simulations for optical Kerr effect of TIP4P/2005 water in liquid and supercooled states
(2018-11-01)
Tang, Ping-Han
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Wu, Ten-Ming
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物理研究所
;
Institute of Physics
The optical Kerr effect (OKE) spectroscopy measured with heterodyne detection (HD) is a useful tool to provide information regarding intermolecular vibrations and structural relaxations in liquid water. Recently, the measurements of the OKE spectroscopy have been extended to the supercooled regime of water. Though the measured results can be well described by using a phenomenological model, the time-resolved OKE spectroscopy of liquid and supercooled water still need a comprehensive understanding. In this paper, we investigated the OKE nuclear response functions of this peculiar liquid and their reduced spectral densities by performing molecular dynamics simulations with the TIP4P/2005 water model. The collective polarizability of water was computed via a dipolar induction scheme, which involves the intrinsic polarizability and the first-order hyperpolarizability tensor of water molecule. Our simulation results were qualitatively consistent with the HD-OKE experimental observations for displaying that the polarizability anisotropy relaxation of supercooled water in the high density liquid phase was fragile-like by following a stretching exponential decay with an exponent beta(s) insensitive to temperature and the temperature dependence of the relaxation time exhibited a power-law divergence at a singular temperature T-s with a critical exponent gamma(s). Indicated by our quantitative results, T-s was predominately determined by the structural arrest, but beta(s) and gamma(s) were not only related to the structural relaxation but also influenced by the collective polarizability of the liquid. For all investigations, the effects due to the first-order hyperpolarizability tensor were examined. (C) 2018 Elsevier B.V. All rights reserved.
Reorientation of OH-group connecting bifurcated H-bond acceptors in liquid water
(2018-10-16)
Tang, Ping-Han
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Fan, Yi-Yao
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Hsu, Wei-Lin
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Wu, Ten-Ming
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物理研究所
The OH-group connecting bifurcated H-bond (HB) acceptors is one case of the three-centered HBs found in physical and biological systems. An example is the transition state of molecular reorientation jump mechanism in liquid water, ionic solutions, and hydration shells of biomolecules. In this Letter, the orientational time correlation functions of these special OH-groups in water were studied by performing molecular dynamics simulations and using the normal-mode approach developed recently. The reorientation angle of the OH-group before entering the diffusive regime can be estimated through the plateau in the mean-square rotational displacement calculated by several methods.