19,330 research outputs found
Role of the first coordination shell in determining the equilibrium structure and dynamics of simple liquids
The traditional view that the physical properties of a simple liquid are
determined primarily by its repulsive forces was recently challenged by
Berthier and Tarjus, who showed that in some cases ignoring the attractions
leads to large errors in the dynamics [L. Berthier and G. Tarjus, Phys. Rev.
Lett. 103, 170601 (2009); J. Chem. Phys. 134, 214503 (2011)]. We present
simulations of the standard Lennard-Jones liquid at several condensed-fluid
state points, including a fairly low density state and a very high density
state, as well as simulations of the Kob-Andersen binary Lennard-Jones mixture
at several temperatures. By varying the range of the forces, results for the
thermodynamics, dynamics, and structure show that the determining factor for
getting the correct statics and dynamics is not whether or not the attractive
forces {\it per se} are included in the simulations. What matters is whether or
not interactions are included from all particles within the first coordination
shell (FCS) - the attractive forces can thus be ignored, but only at extremely
high densities. The recognition of the importance of a local shell in condensed
fluids goes back to van der Waals; our results confirm this idea and thereby
the basic picture of the old hole- and cell theories for simple condensed
fluids
The Individual and Collective Effects of Exact Exchange and Dispersion Interactions on the Ab Initio Structure of Liquid Water
In this work, we report the results of a series of density functional theory
(DFT) based ab initio molecular dynamics (AIMD) simulations of ambient liquid
water using a hierarchy of exchange-correlation (XC) functionals to investigate
the individual and collective effects of exact exchange (Exx), via the PBE0
hybrid functional, non-local vdW/dispersion interactions, via a fully
self-consistent density-dependent dispersion correction, and approximate
nuclear quantum effects (aNQE), via a 30 K increase in the simulation
temperature, on the microscopic structure of liquid water. Based on these AIMD
simulations, we found that the collective inclusion of Exx, vdW, and aNQE as
resulting from a large-scale AIMD simulation of (HO) at the
PBE0+vdW level of theory, significantly softens the structure of ambient liquid
water and yields an oxygen-oxygen structure factor, , and
corresponding oxygen-oxygen radial distribution function, , that
are now in quantitative agreement with the best available experimental data.
This level of agreement between simulation and experiment as demonstrated
herein originates from an increase in the relative population of water
molecules in the interstitial region between the first and second coordination
shells, a collective reorganization in the liquid phase which is facilitated by
a weakening of the hydrogen bond strength by the use of the PBE0 hybrid XC
functional, coupled with a relative stabilization of the resultant disordered
liquid water configurations by the inclusion of non-local vdW/dispersion
interactions
Ab initio theory and modeling of water
Water is of the utmost importance for life and technology. However, a
genuinely predictive ab initio model of water has eluded scientists. We
demonstrate that a fully ab initio approach, relying on the strongly
constrained and appropriately normed (SCAN) density functional, provides such a
description of water. SCAN accurately describes the balance among covalent
bonds, hydrogen bonds, and van der Waals interactions that dictates the
structure and dynamics of liquid water. Notably, SCAN captures the density
difference between water and ice I{\it h} at ambient conditions, as well as
many important structural, electronic, and dynamic properties of liquid water.
These successful predictions of the versatile SCAN functional open the gates to
study complex processes in aqueous phase chemistry and the interactions of
water with other materials in an efficient, accurate, and predictive, ab initio
manner
Pairing of 1-hexyl-3-methylimidazolium and tetrafluoroborate ions in n-pentanol
Molecular dynamics simulations are obtained and analyzed to study pairing of
1-hexyl-3-methylimidazolium and tetrafluoroborate ions in n-pentanol, in
particular by evaluating the potential-of-mean-force between counter ions. The
present molecular model and simulation accurately predicts the dissociation
constant Kd in comparison to experiment, and thus the behavior and magnitudes
for the ion-pair pmf at molecular distances, even though the dielectric
constant of the simulated solvent differs from the experimental value by about
30%. A naive dielectric model does not capture molecule structural effects such
as multiple conformations and binding geometries of the Hmim+ and BF4-
ion-pairs. Mobilities identify multiple time-scale effects in the
autocorrelation of the random forces on the ions, and specifically a slow,
exponential time-decay of those long-ranged forces associated here with
dielectric friction effects.Comment: 5 pages, 7 figures. V2: Figs. 4 & 7 redrawn for better visual clarity
with log-scales. No change in results. In press J. Chem. Phys. 201
Probing defects and correlations in the hydrogen-bond network of ab initio water
The hydrogen-bond network of water is characterized by the presence of
coordination defects relative to the ideal tetrahedral network of ice, whose
fluctuations determine the static and time-dependent properties of the liquid.
Because of topological constraints, such defects do not come alone, but are
highly correlated coming in a plethora of different pairs. Here we discuss in
detail such correlations in the case of ab initio water models and show that
they have interesting similarities to regular and defective solid phases of
water. Although defect correlations involve deviations from idealized
tetrahedrality, they can still be regarded as weaker hydrogen bonds that retain
a high degree of directionality. We also investigate how the structure and
population of coordination defects is affected by approximations to the
inter-atomic potential, finding that in most cases, the qualitative features of
the hydrogen bond network are remarkably robust
Molecular structural order and anomalies in liquid silica
The present investigation examines the relationship between structural order,
diffusivity anomalies, and density anomalies in liquid silica by means of
molecular dynamics simulations. We use previously defined orientational and
translational order parameters to quantify local structural order in atomic
configurations. Extensive simulations are performed at different state points
to measure structural order, diffusivity, and thermodynamic properties. It is
found that silica shares many trends recently reported for water [J. R.
Errington and P. G. Debenedetti, Nature 409, 318 (2001)]. At intermediate
densities, the distribution of local orientational order is bimodal. At fixed
temperature, order parameter extrema occur upon compression: a maximum in
orientational order followed by a minimum in translational order. Unlike water,
however, silica's translational order parameter minimum is broad, and there is
no range of thermodynamic conditions where both parameters are strictly
coupled. Furthermore, the temperature-density regime where both structural
order parameters decrease upon isothermal compression (the structurally
anomalous regime) does not encompass the region of diffusivity anomalies, as
was the case for water.Comment: 30 pages, 8 figure
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