251 |
|
\caption{The external temperature and pressure bath interacts only |
252 |
|
with those atoms on the convex hull (grey surface). The hull is |
253 |
|
computed dynamically at each time step, and molecules can move |
254 |
< |
between the interior (Newtonian) region and the Langevin hull.} |
254 |
> |
between the interior (Newtonian) region and the Langevin Hull.} |
255 |
|
\label{fig:hullSample} |
256 |
|
\end{figure} |
257 |
|
|
266 |
|
potential energy. For atoms on the exterior of the cluster |
267 |
|
(i.e. those that occupy one of the vertices of the convex hull), the |
268 |
|
equation of motion is modified with an external force, ${\mathbf |
269 |
< |
F}_i^{\mathrm ext}$, |
269 |
> |
F}_i^{\mathrm ext}$: |
270 |
|
\begin{equation} |
271 |
|
m_i \dot{\mathbf v}_i(t)=-{\mathbf \nabla}_i U + {\mathbf F}_i^{\mathrm ext}. |
272 |
|
\end{equation} |
412 |
|
To test the new method, we have carried out simulations using the |
413 |
|
Langevin Hull on: 1) a crystalline system (gold nanoparticles), 2) a |
414 |
|
liquid droplet (SPC/E water),\cite{Berendsen1987} and 3) a |
415 |
< |
heterogeneous mixture (gold nanoparticles in a water droplet). In each |
416 |
< |
case, we have computed properties that depend on the external applied |
417 |
< |
pressure. Of particular interest for the single-phase systems is the |
418 |
< |
isothermal compressibility, |
415 |
> |
heterogeneous mixture (gold nanoparticles in an SPC/E water droplet). In each case, we have computed properties that depend on the external applied pressure. Of particular interest for the single-phase systems is the isothermal compressibility, |
416 |
|
\begin{equation} |
417 |
|
\kappa_{T} = -\frac{1}{V} \left ( \frac{\partial V}{\partial P} \right |
418 |
|
)_{T}. |
421 |
|
|
422 |
|
One problem with eliminating periodic boundary conditions and |
423 |
|
simulation boxes is that the volume of a three-dimensional point cloud |
424 |
< |
is not well-defined. In order to compute the compressibility of a |
424 |
> |
is not well-defined. In order to compute the compressibility of a |
425 |
|
bulk material, we make an assumption that the number density, $\rho = |
426 |
< |
\frac{N}{V}$, is uniform within some region of the point cloud. The |
426 |
> |
\frac{N}{V}$, is uniform within some region of the point cloud. The |
427 |
|
compressibility can then be expressed in terms of the average number |
428 |
|
of particles in that region, |
429 |
|
\begin{equation} |
430 |
|
\kappa_{T} = -\frac{1}{N} \left ( \frac{\partial N}{\partial P} \right |
431 |
< |
)_{T} |
431 |
> |
)_{T}. |
432 |
|
\label{eq:BMN} |
433 |
|
\end{equation} |
434 |
< |
The region we used is a spherical volume of 10 \AA\ radius centered in |
434 |
> |
The region we used is a spherical volume of 20 \AA\ radius centered in |
435 |
|
the middle of the cluster. $N$ is the average number of molecules |
436 |
|
found within this region throughout a given simulation. The geometry |
437 |
|
and size of the region is arbitrary, and any bulk-like portion of the |
518 |
|
temperature respond to the Langevin Hull for nanoparticles that were |
519 |
|
initialized far from the target pressure and temperature. As |
520 |
|
expected, the rate at which thermal equilibrium is achieved depends on |
521 |
< |
the total surface area of the cluter exposed to the bath as well as |
521 |
> |
the total surface area of the cluster exposed to the bath as well as |
522 |
|
the bath viscosity. Pressure that is applied suddenly to a cluster |
523 |
|
can excite breathing vibrations, but these rapidly damp out (on time |
524 |
< |
scales of 30-50 ps). |
524 |
> |
scales of 30 -- 50 ps). |
525 |
|
|
526 |
|
\subsection{Compressibility of SPC/E water clusters} |
527 |
|
|
585 |
|
fixed region, |
586 |
|
\begin{equation} |
587 |
|
\kappa_{T} = \frac{\left \langle N^{2} \right \rangle - \left \langle |
588 |
< |
N \right \rangle ^{2}}{N \, k_{B} \, T}, |
588 |
> |
N \right \rangle ^{2}}{N \, k_{B} \, T}. |
589 |
|
\label{eq:BMNfluct} |
590 |
|
\end{equation} |
591 |
|
Thus, the compressibility of each simulation can be calculated |
613 |
|
hydrophobic boundary, or orientational or radial constraints. |
614 |
|
Therefore, the orientational correlations of the molecules in water |
615 |
|
clusters are of particular interest in testing this method. Ideally, |
616 |
< |
the water molecules on the surfaces of the clusterss will have enough |
616 |
> |
the water molecules on the surfaces of the clusters will have enough |
617 |
|
mobility into and out of the center of the cluster to maintain |
618 |
|
bulk-like orientational distribution in the absence of orientational |
619 |
|
and radial constraints. However, since the number of hydrogen bonding |
643 |
|
\includegraphics[width=\linewidth]{pAngle} |
644 |
|
\caption{Distribution of $\cos{\theta}$ values for molecules on the |
645 |
|
interior of the cluster (squares) and for those participating in the |
646 |
< |
convex hull (circles) at a variety of pressures. The Langevin hull |
646 |
> |
convex hull (circles) at a variety of pressures. The Langevin Hull |
647 |
|
exhibits minor dewetting behavior with exposed oxygen sites on the |
648 |
|
hull water molecules. The orientational preference for exposed |
649 |
|
oxygen appears to be independent of applied pressure. } |
655 |
|
orientations. Molecules included in the convex hull show a slight |
656 |
|
preference for values of $\cos{\theta} < 0.$ These values correspond |
657 |
|
to molecules with oxygen directed toward the exterior of the cluster, |
658 |
< |
forming a dangling hydrogen bond acceptor site. |
658 |
> |
forming dangling hydrogen bond acceptor sites. |
659 |
|
|
660 |
< |
In the absence of an electrostatic contribution from the exterior |
661 |
< |
bath, the orientational distribution of water molecules included in |
662 |
< |
the Langevin Hull will slightly resemble the distribution at a neat |
663 |
< |
water liquid/vapor interface. Previous molecular dynamics simulations |
664 |
< |
of SPC/E water \cite{Taylor1996} have shown that molecules at the |
668 |
< |
liquid/vapor interface favor an orientation where one hydrogen |
669 |
< |
protrudes from the liquid phase. This behavior is demonstrated by |
670 |
< |
experiments \cite{Du1994} \cite{Scatena2001} showing that |
671 |
< |
approximately one-quarter of water molecules at the liquid/vapor |
672 |
< |
interface form dangling hydrogen bonds. The negligible preference |
673 |
< |
shown in these cluster simulations could be removed through the |
674 |
< |
introduction of an implicit solvent model, which would provide the |
675 |
< |
missing electrostatic interactions between the cluster molecules and |
676 |
< |
the surrounding temperature/pressure bath. |
660 |
> |
The orientational preference exhibited by water molecules on the hull |
661 |
> |
is significantly weaker than the preference caused by an explicit |
662 |
> |
hydrophobic bounding potential. Additionally, the Langevin Hull does |
663 |
> |
not require that the orientation of any molecules be fixed in order to |
664 |
> |
maintain bulk-like structure, even near the cluster surface. |
665 |
|
|
666 |
< |
The orientational preference exhibited by hull molecules in the |
667 |
< |
Langevin hull is significantly weaker than the preference caused by an |
668 |
< |
explicit hydrophobic bounding potential. Additionally, the Langevin |
669 |
< |
Hull does not require that the orientation of any molecules be fixed |
670 |
< |
in order to maintain bulk-like structure, even at the cluster surface. |
666 |
> |
Previous molecular dynamics simulations of SPC/E liquid / vapor |
667 |
> |
interfaces using periodic boundary conditions have shown that |
668 |
> |
molecules on the liquid side of interface favor a similar orientation |
669 |
> |
where oxygen is directed away from the bulk.\cite{Taylor1996} These |
670 |
> |
simulations had well-defined liquid and vapor phase regions |
671 |
> |
equilibrium and it was observed that {\it vapor} molecules generally |
672 |
> |
had one hydrogen protruding from the surface, forming a dangling |
673 |
> |
hydrogen bond donor. Our water clusters do not have a true vapor |
674 |
> |
region, but rather a few transient molecules that leave the liquid |
675 |
> |
droplet (and which return to the droplet relatively quickly). |
676 |
> |
Although we cannot obtain an orientational preference of vapor phase |
677 |
> |
molecules in a Langevin Hull simulation, but we do agree with previous |
678 |
> |
estimates of the orientation of {\it liquid phase} molecules at the |
679 |
> |
interface. |
680 |
|
|
681 |
|
\subsection{Heterogeneous nanoparticle / water mixtures} |
682 |
|
|
683 |
|
To further test the method, we simulated gold nanopartices ($r = 18$ |
684 |
< |
\AA) solvated by explicit SPC/E water clusters using the Langevin |
685 |
< |
hull. This was done at pressures of 1, 2, 5, 10, 20, 50, 100 and 200 atm |
686 |
< |
in order to observe the effects of pressure on the ordering of water |
687 |
< |
ordering at the surface. In Fig. \ref{fig:RhoR} we show the density |
688 |
< |
of water adjacent to the surface as a function of pressure, as well as |
689 |
< |
the orientational ordering of water at the surface of the |
690 |
< |
nanoparticle. |
684 |
> |
\AA) solvated by explicit SPC/E water clusters using a model for the |
685 |
> |
gold / water interactions that has been used by Dou {\it et. al.} for |
686 |
> |
investigating the separation of water films near hot metal |
687 |
> |
surfaces.\cite{ISI:000167766600035} The Langevin Hull was used to |
688 |
> |
sample pressures of 1, 2, 5, 10, 20, 50, 100 and 200 atm, while all |
689 |
> |
simulations were done at a temperature of 300 K. At these |
690 |
> |
temperatures and pressures, there is no observed separation of the |
691 |
> |
water film from the surface. |
692 |
|
|
693 |
< |
\begin{figure} |
693 |
> |
In Fig. \ref{fig:RhoR} we show the density of water and gold as a |
694 |
> |
function of the distance from the center of the nanoparticle. Higher |
695 |
> |
applied pressures appear to destroy structural correlations in the |
696 |
> |
outermost monolayer of the gold nanoparticle as well as in the water |
697 |
> |
at the near the metal / water interface. Simulations at increased |
698 |
> |
pressures exhibit significant overlap of the gold and water densities, |
699 |
> |
indicating a less well-defined interfacial surface. |
700 |
|
|
701 |
< |
\caption{interesting plot showing cluster behavior} |
701 |
> |
\begin{figure} |
702 |
> |
\includegraphics[width=\linewidth]{RhoR} |
703 |
> |
\caption{Density profiles of gold and water at the nanoparticle |
704 |
> |
surface. Each curve has been normalized by the average density in |
705 |
> |
the bulk-like region available to the corresponding material. Higher applied pressures |
706 |
> |
de-structure both the gold nanoparticle surface and water at the |
707 |
> |
metal/water interface.} |
708 |
|
\label{fig:RhoR} |
709 |
|
\end{figure} |
710 |
|
|
711 |
< |
At higher pressures, problems with the gold - water interaction |
712 |
< |
potential became apparent. The model we are using (due to Spohr) was |
713 |
< |
intended for relatively low pressures; it utilizes both shifted Morse |
714 |
< |
and repulsive Morse potentials to model the Au/O and Au/H |
715 |
< |
interactions, respectively. The repulsive wall of the Morse potential |
716 |
< |
does not diverge quickly enough at short distances to prevent water |
717 |
< |
from diffusing into the center of the gold nanoparticles. This |
718 |
< |
behavior is likely not a realistic description of the real physics of |
719 |
< |
the situation. A better model of the gold-water adsorption behavior |
720 |
< |
appears to require harder repulsive walls to prevent this behavior. |
711 |
> |
At even higher pressures (500 atm and above), problems with the metal |
712 |
> |
- water interaction potential became quite clear. The model we are |
713 |
> |
using appears to have been parameterized for relatively low pressures; |
714 |
> |
it utilizes both shifted Morse and repulsive Morse potentials to model |
715 |
> |
the Au/O and Au/H interactions, respectively. The repulsive wall of |
716 |
> |
the Morse potential does not diverge quickly enough at short distances |
717 |
> |
to prevent water from diffusing into the center of the gold |
718 |
> |
nanoparticles. This behavior is likely not a realistic description of |
719 |
> |
the real physics of the situation. A better model of the gold-water |
720 |
> |
adsorption behavior appears to require harder repulsive walls to |
721 |
> |
prevent this behavior. |
722 |
|
|
723 |
|
\section{Discussion} |
724 |
|
\label{sec:discussion} |
793 |
|
|
794 |
|
For a large number of atoms on a moderately parallel machine, the |
795 |
|
total costs are dominated by the computations of the individual hulls, |
796 |
< |
and communication of these hulls to create the Langevin hull sees roughly |
796 |
> |
and communication of these hulls to create the Langevin Hull sees roughly |
797 |
|
linear speed-up with increasing processor counts. |
798 |
|
|
799 |
|
\section*{Acknowledgments} |