Comparative study of cluster Ag17Cu2 by instantaneous normal mode analysis and by isothermal Brownian-type molecular dynamics simulation

Hsu

et al. 2012

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This paper studies the melting behavior of Ag(14) cluster employing the instantaneous normal mode (INM) analysis that was previously developed for bimetallic cluster Ag(17)Cu(2). 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 Ag(17)Cu(2) 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 τ(T) defined in the context of the INM method. Two general approaches are proposed to calculate τ(T). In one, τ(T) is defined in terms of the INM vibrational density of states; in another, τ(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 τ(T) calculated and also the consistent interpretation of the melting behavior with the specific heat data. The order parameter τ(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.

Section: Introductionsupporting

confidence: 80%

“…Beginning at the LES, the cluster at a finite temperature can be prepared by the MD run, in which the cluster temperature is raised gradually from zero. 24,29 In all of these simulations, atoms at a finite temperature are traced by their labels which are tagged at their LES.…”

Section: A Simulationmentioning

confidence: 99%

Melting behavior of Ag14 cluster: An order parameter by instantaneous normal modes

Hsu

et al. 2012

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“…By contrast, the second-cumulant predictions obtained using the stable INMs were in complete agreement with the simulation results only over timescales shorter than t min ; this was consistent with past observations for other TCFs predicted using the stable INMs. 77, 88, 89 However, although not in complete agreement with the simulation results, in general, the predictions obtained using the stable INMs for the orientational TCFs captured the behavior of the simulation results up to approximately 0.1 ps; during this period, the recurrence was included.…”

Section: A Global Moleculesmentioning

confidence: 80%

“…(27) In the stable-INM approximation, in which the imaginary branch of the INM spectrum is neglected, the AVAF power spectrum D ν (ω) is further approximated as the rotational stable-INM spectrum D I NM,s ν (ω), which is renormalized. 59,76,77 Normally, the lower the fraction of imaginary INMs in a system is, the more accurate the stable-INM approximation. Low fractions of imaginary INMs are found in systems at low temperatures or those with H-bonds, such as liquid water.…”

Section: |mentioning

confidence: 99%

Local structural effects on orientational relaxation of OH-bond in liquid water over short to intermediate timescales

Lin

2014

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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.

“…32,33 Nowadays, the INM theory has been developed as an analytical tool to study the vibrational modes in disordered systems and some results by this analysis have been reported recently. [34][35][36][37][38][39][40] In general, the density of states (DOS) of INMs can be related to the vibrational spectrum observed by experiments. On the other hand, with inverse participation ratio (IPR), 41 the localization of INMs can be studied.…”

Section: Introductionmentioning

confidence: 99%

Instantaneous normal mode analysis for intermolecular and intramolecular vibrations of water from atomic point of view

Chen

2013

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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.