arXiv subjects

Charusita Chakravarty

Publications and source records attributed to Charusita Chakravarty.

12 recordsLinked to original sources

arXiv2011

The Rise and Fall of Anomalies in Tetrahedral Liquids

The thermodynamic liquid-state anomalies and associated structural changes of the Stillinger-Weber family of liquids are mapped out as a function of the degree of tetrahedrality of the interaction potential, focusing in particular on tetrahedrality values suitable for modeling C, H2O, Si, Ge and Sn. We show that the density anomaly, associated with a rise in molar volume on isobaric cooling, emerges at intermediate tetrahedralities (e.g. Ge, Si and H2O) but is absent in the low (e.g. Sn) and high (e.g. C) tetrahedrality liquids. The rise in entropy on isothermal compression associated with the density anomaly is related to the structural changes in the liquid using the pair correlation entropy. An anomalous increase in the heat capacity on isobaric cooling exists at high tetrahedralities but is absent at low tetrahedralities (e.g. Sn). Structurally, this heat capacity anomaly originates in a sharp rise in the fraction of four-coordinated particles and local tetrahedral order in the liquid as its structure approaches that of the tetrahedral crystal.

cond-mat.soft↗

arXiv2011

Core-softened Fluids, Water-like Anomalies and the Liquid-Liquid Critical Points

Molecular dynamics simulations are used to examine the relationship between water-like anomalies and the liquid-liquid critical point in a family of model fluids with multi-Gaussian, core-softened pair interactions. The core-softened pair interactions have two length scales, such that the longer length scale associated with a shallow, attractive well is kept constant while the shorter length scale associated with the repulsive shoulder is varied from an inflexion point to a minimum of progressively increasing depth. As the shoulder well depth increases, the pressure required to form the high density liquid decreases and the temperature up to which the high-density liquid is stable increases, resulting in the shift of the liquid-liquid critical point to much lower pressures and higher temperatures. The pair correlation entropy is computed to show that the excess entropy anomaly diminishes when the shoulder well depth increases. Excess entropy scaling of diffusivity in this class of fluids is demonstrated, showing that decreasing strength of the excess entropy anomaly with increasing shoulder depth results in the progressive loss of water-like thermodynamic, structural and transport anomalies. Instantaneous normal mode analysis was used to index the overall curvature distribution of the fluid and the fraction of imaginary frequency modes was shown to correlate well with the anomalous behaviour of the diffusivity and the pair correlation entropy. The results suggest in the case of core-softened potentials, in addition to the presence of two length scales, energetic and entropic effects associated with local minima and curvatures of the pair interaction play an important role in determining the presence of water-like anomalies and the liquid-liquid phase transition.

cond-mat.soft↗

arXiv2010

Relationship between Structure, Entropy and Diffusivity in Water and Water-like Liquids

Anomalous behaviour of the excess entropy ($S_e$) and the associated scaling relationship with diffusivity are compared in liquids with very different underlying interactions but similar water-like anomalies: water (SPC/E and TIP3P models), tetrahedral ionic melts (SiO$_2$ and BeF$_2$) and a fluid with core-softened, two-scale ramp (2SRP) interactions. We demonstrate the presence of an excess entropy anomaly in the two water models. Using length and energy scales appropriate for onset of anomalous behaviour, the density range of the excess entropy anomaly is shown to be much narrower in water than in ionic melts or the 2SRP fluid. While the reduced diffusivities ($D^*$) conform to the excess entropy scaling relation, $D^* =A\exp (αS_e)$ for all the systems (Y. Rosenfeld, Phys. Rev. A {\bf 1977}, {\it 15}, 2545), the exponential scaling parameter, $α$, shows a small isochore-dependence in the case of water. Replacing $S_e$ by pair correlation-based approximants accentuates the isochore-dependence of the diffusivity scaling. Isochores with similar diffusivity scaling parameters are shown to have the temperature dependence of the corresponding entropic contribution. The relationship between diffusivity, excess entropy and pair correlation approximants to the excess entropy are very similar in all the tetrahedral liquids.

cond-mat.soft↗

arXiv2010

Entropy, diffusivity and the energy landscape of a water-like fluid

Molecular dynamics simulations and instantaneous normal mode (INM) analysis of a fluid with core-softened pair interactions and water-like liquid-state anomalies are performed to obtain an understanding of the relationship between thermodynamics, transport properties and the poten- tial energy landscape. Rosenfeld-scaling of diffusivities with the thermodynamic excess and pair correlation entropy is demonstrated for this model. The INM spectra are shown to carry infor- mation about the dynamical consequences of the interplay between length scales characteristic of anomalous fluids, such as bimodality of the real and imaginary branches of the frequency distribu- tion. The INM spectral information is used to partition the liquid entropy into two contributions associated with the real and imaginary frequency modes; only the entropy contribution from the imaginary branch captures the non-monotonic behaviour of the excess entropy and diffusivity in the anomalous regime of the fluid.

cond-mat.soft↗

arXiv2008

Multiple Time-Scale Behaviour and Network Dynamics in Liquid Methanol

Canonical ensemble molecular dynamics simulations of liquid methanol, modeled using a rigid-body, pair-additive potential, are used to compute static distributions and temporal correlations of tagged molecule potential energies as a means of characterising the liquid state dynamics. The static distribution of tagged molecule potential energies shows a clear multimodal structure with three distinct peaks, similar to those observed previously in water and liquid silica. The multimodality is shown to originate from electrostatic effects, but not from local, hydrogen-bond interactions. An interesting outcome of this study is the remarkable similarity in the tagged potential energy power spectra of methanol, water and silica, despite the differences in the underlying interactions and the dimensionality of the network. All three liquids show a distinct multiple time scale (MTS) regime with a 1/f dependence with a clear positive correlation between the scaling exponent alpha and the diffusivity. The low-frequency limit of the MTS regime is determined by the frequency of crossover to white noise behaviour which occurs at approximately 0.1 cm{-1} in the case of methanol under standard temperature and pressure conditions. The power spectral regime above 200 cm{-1} in all three systems is dominated by resonances due to localised vibrations, such as librations. The correlation between $α$ and the diffusivity in all three liquids appears to be related to the strength of the coupling between the localised motions and the larger length/time-scale network reorganizations. Thus the time scales associated with network reorganization dynamics appear to be qualitatively similar in these systems, despite the fact that water and silica both display diffusional anomalies but methanol does not.

cond-mat.soft↗

arXiv2008

Estimating Entropy of Liquids from Atom-Atom Radial Distribution Functions: Silica, Beryllium Fluoride and Water

Molecular dynamics simulations of water, liquid beryllium fluoride and silica melt are used to study the accuracy with which the entropy of ionic and molecular liquids can be estimated from atom-atom radial distribution function data. All three systems are known to display similar liquid-state thermodynamic and kinetic anomalies due to a region of anomalous excess entropy behaviour where entropy rises on isothermal compression. The pair correlation entropy is demonstrated to be sufficiently accurate that the density-temperature regime of anomalous behaviour as well as the strength of the entropy anomaly can be predicted reliably for both ionic melts as well as different rigid-body pair potentials for water. Errors in the total thermodynamic entropy for ionic melts due to the pair correlation approximation are of the order of 10% or less for most state points but can be significantly larger in the anomalous regime at very low temperatures. In the case of water, as expected given the rigid-body constraints for a molecular liquids, the pair correlation approximation causes significantly larger errors, between 20 and 30%, for most state points. Comparison of the excess entropy, Se, of ionic melts with the pair correlation entropy, S2, shows that the temperature dependence of Se is well described by T ??2=5 scaling across both the normal and anomalous regimes, unlike in the case of S2. As a function of density, the Se(rho) curves shows only a single maximum while the S2(rho) curves show both a maximum and a minimum. These differences in the behaviour of S2 and Se are due to the fact that the residual multiparticle entropy, delta(S) = Se - S2, shows a strong negative correlation with tetrahedral order in the anomalous regime.

cond-mat.soft↗

arXiv2006

Diffusional Anomaly and network dynamics in liquid silica

The present study applies the power spectral analysis technique to understand the diffusional anomaly in liquid silica, modeled using the BKS potential. Molecular dynamics simulations have been carried out to show that power spectrum of tagged particle potential energy of silica shows a regime with 1/f dependence on frequency f which is the characteristic signature of multiple time-scale behaviour in networks. As demonstrated earlier in the case of water (J. Chem. Phys., 122, 104507 (2005)), the variations in the mobility associated with the diffusional anomaly are mirrored in the scaling exponent alpha associated with this multiple time-scale behaviour. Our results indicate that in the anomalous regime, as the local tetrahedral order decreases with temperature or pressure, the coupling of local modes to network reorganisations increases and so does the diffusivity. This symmetry-dependence of the vibrational couplings is responsible for the connection between the structural and diffusional anomalies.

cond-mat.soft↗

arXiv2006

Excess entropy, Diffusivity and Structural Order in liquids with water-like anomalies

The excess entropy, Se, defined as the difference between the entropies of the liquid and the ideal gas under identical density and temperature conditions, is shown to be the critical quantity connecting the structural, diffusional and density anomalies in water-like liquids. Based on simulations of silica and the two-scale ramp liquids, water-like density and diffusional anomalies can be seen as consequences of a characteristic non-monotonic density dependence of Se. The relationship between excess entropy, the order metrics and the structural anomaly can be understood using a pair correlation approximation to Se.

cond-mat.soft↗

arXiv2004

Spectral Signatures of the Diffusional Anomaly in Water

Analysis of power spectrum profiles for various tagged particle quantities in bulk SPC/E water is used to demonstrate that variations in mobility associated with the diffusional anomaly are mirrored in the exponent of the \onebyf\ region. Monitoring of \onebyf behaviour is shown to be a simple and direct method for linking phenomena on three distinctive length and time scales: the local molecular environment, hydrogen bond network reorganisations and the diffusivity. The results indicate that experimental studies of supercooled water to probe the density dependence of $1/f^α$ spectral features, or equivalent stretched exponential behaviour in time-correlation functions, will be of interest.

cond-mat.soft↗

arXiv2004

Effect of the Berendsen thermostat on dynamical properties of water

The effect of the Berendsen thermostat on the dynamical properties of bulk SPC/E water is tested by generating power spectra associated with fluctuations in various observables. The Berendsen thermostat is found to be very effective in preserving temporal correlations in fluctuations of tagged particle quantities over a very wide range of frequencies. Even correlations in fluctuations of global properties, such as the total potential energy, are well-preserved for time periods shorter than the thermostat time constant. Deviations in dynamical behaviour from the microcanonical limit do not, however, always decrease smoothly with increasing values of the thermostat time constant but may be somewhat larger for some intermediate values of $τ_B$, specially in the supercooled regime, which are similar to time scales for slow relaxation processes in bulk water.

cond-mat.soft↗

arXiv2002

Quasi-saddles of Liquids: Computational Study of a bulk Lennard-Jones system

Quasi-saddles or inherent saddles of the potential energy surface, $U$, of a liquid are defined as configurations which correspond to absolute minima of the pseudo-potential surface, $W =\wf$, as identified by a multi-dimensional minimisation procedure. The sensitivity of statistical properties of inherent saddles to the convergence criteria of the minimisation procedure is investigated using, as a test system, a simple liquid bound by a quadratically shifted Lennard-Jones pair potential with continuous zeroth, first and second derivatives at the cut-off distance. The variation in statistical properties of saddles is studied over a range of error tolerances spanning five orders of magnitude. The largest value of the tolerance corresponds to that used for the unshifted LJ liquids in a previous work (J. Chem. Phys. {\bf 115}, 8784 (2001)). Based on our results, it is clear that there are no qualitative changes in statistical properties of saddles over this range of error tolerances and even the quantitative changes are small. The lowest magnitude eigenvalue, $| ω_0^2|$, of the Hessian is, however, found to be very sensitive to the tolerance; as the tolerance is decreased, $| ω_0^2|$ is found to show an overall decrease. This indicates that if convergence criteria are not strict, absolute or low-lying minima of $W(\br)$ will be diagnosed as having no inflexion directions. The results also show that it is not possible to set up an unambiguous numerical criterion to further classify the quasi-saddles into true saddles which contain no zero curvature, non-translational normal modes and inflexion points which have one or more zero-curvature normal mode directions.

cond-mat↗

arXiv1998

An ab initio path integral Monte Carlo simulation method for molecules and clusters: application to Li_4 and Li_5^+

A novel method for simulating the statistical mechanics of molecular systems in which both nuclear and electronic degrees of freedom are treated quantum mechanically is presented. The scheme combines a path integral description of the nuclear variables with a first-principles adiabatic description of the electronic structure. The electronic problem is solved for the ground state within a density functional approach, with the electronic orbitals expanded in a localized (Gaussian) basis set. The discretized path integral is computed by a Metropolis Monte Carlo sampling technique on the normal modes of the isomorphic ring-polymer. An effective short-time action correct to order $τ^4$ is used. The validity and performance of the method are tested in two small Lithium clusters, namely Li$_4$ and Li$_5^+$. Structural and electronic properties computed within this fully quantum-mechanical scheme are presented and compared to those obtained within the classical nuclei approximation. Quantum delocalization effects are significant but tunneling turns out to be irrelevant at low temperatures.

physics.comp-ph↗