trunk/nonperiodicVSS/nonperiodicVSS.tex

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\title{A Method for Creating Thermal and Angular Momentum Fluxes in Non-Periodic Systems}

\author{Kelsey M. Stocker}
\author{J. Daniel Gezelter}
\email{gezelter@nd.edu}
\affiliation[University of Notre Dame]{251 Nieuwland Science Hall\\ Department of Chemistry and Biochemistry\\ University of Notre Dame\\ Notre Dame, Indiana 46556}

\begin{document}

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% \author{Kelsey M. Stocker and J. Daniel
%   Gezelter\footnote{Corresponding author. \ Electronic mail:
%     gezelter@nd.edu} \\
%   251 Nieuwland Science Hall, \\
%       Department of Chemistry and Biochemistry,\\
%       University of Notre Dame\\
%       Notre Dame, Indiana 46556}

\date{\today}

\maketitle 

\begin{doublespace}

\begin{abstract}

We have adapted the Velocity Shearing and Scaling Reverse Non-Equilibium Molecular Dynamics (VSS-RNEMD) method for use with aperiodic system geometries. This new method is capable of creating stable temperature and angular velocity gradients in heterogeneous non-periodic systems while conserving total energy and angular momentum. To demonstrate the method, we have computed the thermal conductivities of a gold nanoparticle and water cluster, the shear viscosity of a water cluster, the interfacial thermal conductivity of a solvated gold nanoparticle and the interfacial friction of solvated gold nanostructures.

\end{abstract}

\newpage

%\narrowtext

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% **INTRODUCTION**
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\section{Introduction}

Non-equilibrium Molecular Dynamics (NEMD) methods impose a temperature
or velocity {\it gradient} on a
system,\cite{ASHURST:1975tg,Evans:1982zk,ERPENBECK:1984sp,MAGINN:1993hc,Berthier:2002ij,Evans:2002ai,Schelling:2002dp,PhysRevA.34.1449,JiangHao_jp802942v}
and use linear response theory to connect the resulting thermal or
momentum flux to transport coefficients of bulk materials.  However,
for heterogeneous systems, such as phase boundaries or interfaces, it
is often unclear what shape of gradient should be imposed at the
boundary between materials.

\begin{figure}
\includegraphics[width=\linewidth]{figures/npVSS}
\caption{Schematics of periodic (left) and non-periodic (right)
  Velocity Shearing and Scaling RNEMD. A kinetic energy or momentum
  flux is applied from region B to region A. Thermal gradients are
  depicted by a color gradient. Linear or angular velocity gradients
  are shown as arrows.}
\label{fig:VSS}
\end{figure}

Reverse Non-Equilibrium Molecular Dynamics (RNEMD) methods impose an
unphysical {\it flux} between different regions or ``slabs'' of the
simulation
box.\cite{MullerPlathe:1997xw,ISI:000080382700030,Kuang2010} The
system responds by developing a temperature or velocity {\it gradient}
between the two regions.  The gradients which develop in response to
the applied flux are then related (via linear response theory) to the
transport coefficient of interest. Since the amount of the applied
flux is known exactly, and measurement of a gradient is generally less
complicated, imposed-flux methods typically take shorter simulation
times to obtain converged results. At interfaces, the observed
gradients often exhibit near-discontinuities at the boundaries between
dissimilar materials.  RNEMD methods do not need many trajectories to
provide information about transport properties, and they have become
widely used to compute thermal and mechanical transport in both
homogeneous liquids and
solids~\cite{MullerPlathe:1997xw,ISI:000080382700030,Maginn:2010} as
well as heterogeneous
interfaces.\cite{garde:nl2005,garde:PhysRevLett2009,kuang:AuThl}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% **METHODOLOGY**
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\section{Velocity Shearing and Scaling (VSS) for non-periodic systems}

The VSS-RNEMD approach uses a series of simultaneous velocity shearing
and scaling exchanges between the two slabs.\cite{Kuang2012}
This method imposes energy and momentum conservation constraints while
simultaneously creating a desired flux between the two slabs.  These
constraints ensure that all configurations are sampled from the same
microcanonical (NVE) ensemble.

We have extended the VSS method for use in {\it non-periodic}
simulations, in which the ``slabs'' have been generalized to two
separated regions of space.  These regions could be defined as
concentric spheres (as in figure \ref{fig:VSS}), or one of the regions
can be defined in terms of a dynamically changing ``hull'' comprising
the surface atoms of the cluster.  This latter definition is identical
to the hull used in the Langevin Hull algorithm.

We present here a new set of constraints that are more general than
the VSS constraints.  For the non-periodic variant, the constraints
fix both the total energy and total {\it angular} momentum of the
system while simultaneously imposing a thermal and angular momentum
flux between the two regions.

After each $\Delta t$ time interval, the particle velocities
($\mathbf{v}_i$ and $\mathbf{v}_j$) in the two shells ($A$ and $B$)
are modified by a velocity scaling coefficient ($a$ and $b$) and by a
rotational shearing term ($\mathbf{c}_a$ and $\mathbf{c}_b$).
\begin{displaymath}
\begin{array}{rclcl}
 & \underline{~~~~~~~~\mathrm{scaling}~~~~~~~~} & &
 \underline{\mathrm{rotational~shearing}} \\  \\
\mathbf{v}_i $~~~$\leftarrow & 
  a \left(\mathbf{v}_i - \langle \omega_a
  \rangle \times \mathbf{r}_i\right) & + & \mathbf{c}_a \times \mathbf{r}_i \\
\mathbf{v}_j $~~~$\leftarrow & 
  b  \left(\mathbf{v}_j - \langle \omega_b
  \rangle \times \mathbf{r}_j\right) & + & \mathbf{c}_b \times \mathbf{r}_j
\end{array}
\end{displaymath}
Here $\langle\mathbf{\omega}_a\rangle$ and
$\langle\mathbf{\omega}_b\rangle$ are the instantaneous angular
velocities of each shell, and $\mathbf{r}_i$ is the position of
particle $i$ relative to a fixed point in space (usually the center of
mass of the cluster).  Particles in the shells also receive an
additive ``angular shear'' to their velocities.  The amount of shear
is governed by the imposed angular momentum flux,
$\mathbf{j}_r(\mathbf{L})$,
\begin{eqnarray}
\mathbf{c}_a & = & - \mathbf{j}_r(\mathbf{L}) \cdot
\overleftrightarrow{I_a}^{-1} \Delta t   + \langle \omega_a \rangle \label{eq:bc}\\
\mathbf{c}_b & = & + \mathbf{j}_r(\mathbf{L}) \cdot
\overleftrightarrow{I_b}^{-1}  \Delta t  + \langle \omega_b \rangle \label{eq:bh}
\end{eqnarray}
where $\overleftrightarrow{I}_{\{a,b\}}$ is the moment of inertia tensor for
each of the two shells.

To simultaneously impose a thermal flux ($J_r$) between the shells we
use energy conservation constraints,
\begin{eqnarray}
K_a - J_r \Delta t & = & a^2 \left(K_a - \frac{1}{2}\langle
\omega_a \rangle \cdot \overleftrightarrow{I_a} \cdot \langle \omega_a
\rangle \right) + \frac{1}{2} \mathbf{c}_a \cdot \overleftrightarrow{I_a}
\cdot \mathbf{c}_a \label{eq:Kc}\\
K_b + J_r \Delta t & = & b^2 \left(K_b - \frac{1}{2}\langle
\omega_b \rangle \cdot \overleftrightarrow{I_b} \cdot \langle \omega_b
\rangle \right) + \frac{1}{2} \mathbf{c}_b \cdot \overleftrightarrow{I_b} \cdot \mathbf{c}_b \label{eq:Kh}
\end{eqnarray}
Simultaneous solution of these quadratic formulae for the scaling
coefficients, $a$ and $b$, will ensure that the simulation samples
from the original microcanonical (NVE) ensemble.  Here $K_{\{a,b\}}$
is the instantaneous translational kinetic energy of each shell.  At
each time interval, we solve for $a$, $b$, $\mathbf{c}_a$, and
$\mathbf{c}_b$, subject to the imposed angular momentum flux,
$j_r(\mathbf{L})$, and thermal flux, $J_r$, values.  The new particle
velocities are computed, and the simulation continues. System
configurations after the transformations have exactly the same energy
({\it and} angular momentum) as before the moves.

As the simulation progresses, the velocity transformations can be
performed on a regular basis, and the system will develop a
temperature and/or angular velocity gradient in response to the
applied flux.  Using the slope of the radial temperature or velocity
gradients, it is quite simple to obtain both the thermal conductivity
($\lambda$), interfacial thermal conductance ($G$), or rotational friction coefficients ($\Xi^{rr}$) of any nonperiodic system.

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% **COMPUTATIONAL DETAILS**
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\section{Computational Details}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% NON-PERIODIC DYNAMICS
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\subsection{Dynamics for non-periodic systems}

We have run all tests using the Langevin Hull nonperiodic simulation methodology.\cite{Vardeman2011} The Langevin Hull is especially useful for simulating heterogeneous mixtures of materials with different compressibilities, which are typically problematic for traditional affine transform methods. We have had success applying this method to
several different systems including bare metal nanoparticles, liquid water clusters, and explicitly solvated nanoparticles. Calculated physical properties such as the isothermal compressibility of water and the bulk modulus of gold nanoparticles are in good agreement with previous theoretical and experimental results. The Langevin Hull uses a Delaunay tesselation to create a dynamic convex hull composed of triangular facets with vertices at atomic sites. Atomic sites included in the hull are coupled to an external bath defined by a temperature, pressure and viscosity. Atoms not included in the hull are subject to standard Newtonian dynamics.

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% SIMULATION PROTOCOL
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% 
\subsection{Simulation protocol}

Systems containing liquids were run under moderate pressure ($\sim$ 5 atm) to avoid the formation of a substantial vapor phase. Thermal coupling to the Langevin Hull external bath was turned off to avoid interference with any imposed flux.

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% FORCE FIELD PARAMETERS
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\subsection{Force field parameters}

We have chosen the SPC/E water model for these simulations, which is particularly useful for method validation as there are many values for physical properties from previous simulations available for direct comparison.\cite{Bedrov:2000, Kuang2010}

Gold -- gold interactions are described by the quantum Sutton-Chen (QSC) model.\cite{PhysRevB.59.3527} The QSC parameters are tuned to experimental properties such as density, cohesive energy, and elastic moduli and include zero-point quantum corrections.

Hexane molecules are described by the TraPPE united atom model,\cite{TraPPE-UA.alkanes} which provides good computational efficiency and reasonable accuracy for bulk thermal conductivity values. In this model,
sites are located at the carbon centers for alkyl groups. Bonding
interactions, including bond stretches and bends and torsions, were
used for intra-molecular sites closer than 3 bonds. For non-bonded
interactions, Lennard-Jones potentials were used.  We have previously
utilized both united atom (UA) and all-atom (AA) force fields for
thermal conductivity,\cite{kuang:AuThl,Kuang2012} and since the united
atom force fields cannot populate the high-frequency modes that are
present in AA force fields, they appear to work better for modeling
thermal conductivity.

Gold -- hexane nonbonded interactions are governed by pairwise Lennard-Jones parameters derived from Vlugt \emph{et al}.\cite{vlugt:cpc2007154} They fitted parameters for the interaction between Au and CH$_{\emph x}$ pseudo-atoms based on the effective potential of Hautman and Klein for the Au(111) surface.\cite{hautman:4994}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% THERMAL CONDUCTIVITIES
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\subsection{Thermal conductivities}

Fourier's Law of heat conduction in radial coordinates yields an expression for the heat flow between the concentric spherical RNEMD shells:

\begin{equation}
        q_r = - \frac{4 \pi \lambda (T_b - T_a)}{\frac{1}{r_a} - \frac{1}{r_b}}
\label{eq:Q}
\end{equation}

where $\lambda$ is the thermal conductivity, and $T_{a,b}$ and $r_{a,b}$ are the temperatures and radii of the two RNEMD regions, respectively.

Once a stable thermal gradient has been established between the two regions, the thermal conductivity, $\lambda$, can be calculated using a linear regression of the thermal gradient, $\langle \frac{dT}{dr} \rangle$:

\begin{equation}
        \lambda = \frac{r_a}{r_b} \frac{q_r}{4 \pi \langle \frac{dT}{dr} \rangle}
\label{eq:lambda}
\end{equation}

The rate of heat transfer between the two RNEMD regions is the amount of transferred kinetic energy over the length of the simulation, t

\begin{equation}
        q_r = \frac{KE}{t}
\label{eq:heat}
\end{equation}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% INTERFACIAL THERMAL CONDUCTANCE
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\subsection{Interfacial thermal conductance}

A thermal flux is created using VSS-RNEMD moves, and the temperature in each of the radial shells is recorded. The resulting temperature
profiles are analyzed to yield information about the interfacial thermal
conductance. As the simulation progresses, the VSS moves are performed on a regular basis, and
the system develops a thermal or velocity gradient in response to the applied
flux. We can treat the temperature in each radial shell as discrete, making the thermal conductance, $G$, of each shell the inverse of its Kapitza resistance, $R_K$. To model the thermal conductance across an interface (or multiple interfaces) it is useful to consider the shells as resistors wired in series. The total resistance of the shells is then additive, and the interfacial thermal conductance is the inverse of the total Kapitza resistance. The thermal resistance of each shell is

\begin{equation}
        R_K = \frac{1}{q_r} \Delta T 4 \pi r^2
\label{eq:RK}
\end{equation}

making the total resistance of two neighboring shells

\begin{equation}
        R_{total} = \frac{1}{q_r} \left [ (T_2 - T_1) 4 \pi r^2_1 + (T_3 - T_2) 4 \pi r^2_2 \right ]
\label{eq:Rtotal}
\end{equation}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% INTERFACIAL FRICTION
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\subsection{Interfacial friction}

Analytical solutions for the rotational friction coefficients for a solvated spherical body of radius $r$ under ideal ``stick'' boundary conditions can be estimated using Stokes' law

\begin{equation}
        \Xi^{rr} = 8 \pi \eta r^3
\label{eq:Xistick}.
\end{equation}

where $\eta$ is the dynamic viscosity of the surrounding solvent. An $\eta$ value for UA hexane under these particular temperature and pressure conditions was determined by applying a traditional VSS-RNEMD linear momentum flux to a periodic box of solvent.

For general ellipsoids with semiaxes $a$, $b$, and $c$, Perrin's extension of Stokes' law provides exact solutions for symmetric prolate $(a \geq b = c)$ and oblate $(a < b = c)$ ellipsoids. For simplicity, we define a Perrin Factor, $S$,

\begin{equation}
        S = \frac{2}{\sqrt{a^2 - b^2}} ln \left[ \frac{a + \sqrt{a^2 - b^2}}{b} \right].
\label{eq:S}
\end{equation}

For a prolate ellipsoidal rod, demonstrated here, the rotational resistance tensor $\Xi$ is a $3 \times 3$ diagonal matrix with elements
\begin{equation}
        \Xi^{rr}_a = \frac{32 \pi}{2} \eta \frac{ \left( a^2 - b^2 \right) b^2}{2a - b^2 S}
\label{eq:Xia}
\end{equation}\vspace{-0.45in}\\
\begin{equation}
        \Xi^{rr}_{b,c} = \frac{32 \pi}{2} \eta \frac{ \left( a^4 - b^4 \right)}{ \left( 2a^2 - b^2 \right)S - 2a}.
\label{eq:Xibc}
\end{equation}

The effective rotational friction coefficient at the interface can be extracted from nonperiodic VSS-RNEMD simulations quite easily using the applied torque ($\tau$) and the observed angular velocity of the gold structure ($\omega_{Au}$)

\begin{equation}
        \Xi^{rr}_{\mathit{eff}} = \frac{\tau}{\omega_{Au}}
\label{eq:Xieff}
\end{equation}

The applied torque required to overcome the interfacial friction and maintain constant rotation of the gold is 

\begin{equation}
        \tau = \frac{L}{2 t}
\label{eq:tau}  
\end{equation}

where $L$ is the total angular momentum exchanged between the two RNEMD regions and $t$ is the length of the simulation.

Previous VSS-RNEMD simulations of the interfacial friction of the planar Au(111) / hexane interface have shown that the interface exists within ``slip'' boundary conditions.\cite{Kuang2012} Hu and Zwanzig\cite{Zwanzig} investigated the rotational friction coefficients for spheroids under slip boundary conditions and obtained numerial results for a scaling factor to be applied to $\Xi^{rr}_{\mathit{stick}}$ as a function of $\tau$, the ratio of the shorter semiaxes and the longer semiaxis of the spheroid. For the sphere and prolate ellipsoid shown here, the values of $\tau$ are $1$ and $0.3939$, respectively. According to the values tabulated by Hu and Zwanzig, $\Xi^{rr}_{\mathit{slip}}$ for any sphere approaches $0$, while the ellipsoidal $\Xi^{rr}_{\mathit{slip}}$ is the analytical $\Xi^{rr}_{\mathit{stick}}$ result scaled by a factor of $0.359$ to account for the reduced interfacial friction under ``slip'' boundary conditions.

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% **TESTS AND APPLICATIONS**
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\section{Tests and Applications} 

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% THERMAL CONDUCTIVITIES
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\subsection{Thermal conductivities}

Calculated values for the thermal conductivity of a 40 \AA$~$ radius gold nanoparticle (15707 atoms) at different kinetic energy flux values are shown in Table \ref{table:goldTC}. For these calculations, the hot and cold slabs were excluded from the linear regression of the thermal gradient. The measured linear slope $\langle \frac{dT}{dr} \rangle$ is linearly dependent on the applied kinetic energy flux $J_r$. Calculated thermal conductivity values compare well with previous bulk QSC values of 1.08 -- 1.26 {\footnotesize W / m $\cdot$ K}\cite{Kuang2010}, though still significantly lower than the experimental value of 320 {\footnotesize W / m $\cdot$ K}, as the QSC force field neglects significant electronic contributions to heat conduction. 

% The small increase relative to previous simulated bulk values is due to a slight increase in gold density -- as expected, an increase in density results in higher thermal conductivity values. The increased density is a result of nanoparticle curvature relative to an infinite bulk slab, which introduces surface tension that increases ambient density.

\begin{longtable}{ccc}
\caption{Calculated thermal conductivity of a crystalline gold nanoparticle of radius 40 \AA. Calculations were performed at 300 K and ambient density. Gold-gold interactions are described by the Quantum Sutton-Chen potential.}
\\ \hline \hline
{$J_r$} & {$\langle \frac{dT}{dr} \rangle$} & {$\boldsymbol \lambda$}\\ 
{\footnotesize(kcal / fs $\cdot$ \AA$^{2}$)} & {\footnotesize(K / \AA)} & {\footnotesize(W / m $\cdot$ K)}\\ \hline
3.25$\times 10^{-6}$ & 0.11435 & 1.0143 \\
6.50$\times 10^{-6}$ & 0.2324 & 0.9982 \\
1.30$\times 10^{-5}$ & 0.44922 & 1.0328 \\
3.25$\times 10^{-5}$ & 1.1802 & 0.9828 \\
6.50$\times 10^{-5}$ & 2.339 & 0.9918 \\
\hline
This work & & 1.0040
\\ \hline \hline
\label{table:goldTC}
\end{longtable}

Calculated values for the thermal conductivity of a cluster of 6912 SPC/E water molecules are shown in Table \ref{table:waterTC}. As with the gold nanoparticle thermal conductivity calculations, the RNEMD regions were excluded from the $\langle \frac{dT}{dr} \rangle$ fit. Again, the measured slope is linearly dependent on the applied kinetic energy flux $J_r$. The average calculated thermal conductivity from this work, $0.8841$ {\footnotesize W / m $\cdot$ K}, compares very well to previous nonequilibrium molecular dynamics results\cite{Romer2012, Zhang2005} and experimental values.\cite{WagnerKruse}

\begin{longtable}{ccc}
\caption{Calculated thermal conductivity of a cluster of 6912 SPC/E water molecules. Calculations were performed at 300 K and 5 atm.}
\\ \hline \hline
{$J_r$} & {$\langle \frac{dT}{dr} \rangle$} & {$\boldsymbol \lambda$}\\ 
{\footnotesize(kcal / fs $\cdot$ \AA$^{2}$)} & {\footnotesize(K / \AA)} & {\footnotesize(W / m $\cdot$ K)}\\ \hline
1$\times 10^{-5}$ & 0.38683 & 0.8698 \\
3$\times 10^{-5}$ & 1.1643 & 0.9098 \\
6$\times 10^{-5}$ & 2.2262 & 0.8727 \\
\hline
This work & & 0.8841 \\
Zhang, et al\cite{Zhang2005} & & 0.81 \\
R$\ddot{\mathrm{o}}$mer, et al\cite{Romer2012} & & 0.87 \\
Experiment\cite{WagnerKruse} & & 0.61
\\ \hline \hline
\label{table:waterTC}
\end{longtable}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% INTERFACIAL THERMAL CONDUCTANCE
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\subsection{Interfacial thermal conductance}

Calculated interfacial thermal conductance ($G$) 

\begin{longtable}{ccc}
\caption{Calculated interfacial thermal conductance ($G$) values for gold nanoparticles of varying radii solvated in explicit TraPPE-UA hexane. The nanoparticle $G$ values are compared to previous results for a gold slab in TraPPE-UA hexane, revealing increased interfacial thermal conductance for non-planar interfaces.}
\\ \hline \hline
{Nanoparticle Radius} & {$G$}\\ 
{\footnotesize(\AA)} & {\footnotesize(MW / m$^{2}$ $\cdot$ K)}\\ \hline
20 & {47.1} \\
30 & {45.4} \\
40 & {46.5} \\
slab & {30.2} 
\\ \hline \hline
\label{table:interfacialconductance}
\end{longtable}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% INTERFACIAL FRICTION
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\subsection{Interfacial friction}

Table \ref{table:couple} shows the calculated rotational friction coefficients $\Xi^{rr}$ for spherical gold nanoparticles and a prolate ellipsoidal gold nanorod in TraPPE-UA hexane. An angular momentum flux was applied between the A and B regions defined as the gold structure and hexane molecules beyond a certain radius, respectively. The resulting angular velocity gradient causes the gold structure to rotate about the prescribed axis.
        
\begin{longtable}{lccccc}
\caption{Comparison of rotational friction coefficients under ideal ``slip'' ($\Xi^{rr}_{\mathit{slip}}$) and ``stick'' conditions ($\Xi^{rr}_{\mathit{stick}}$) and effective rotational friction coefficients ($\Xi^{rr}_{\mathit{eff}}$) of gold nanostructures solvated in TraPPE-UA hexane at 230 K. The ellipsoid is oriented with the long axis along the $z$ direction.}
\\ \hline \hline
{Structure} & {Axis of Rotation} & {$\Xi^{rr}_{\mathit{slip}}$} & {$\Xi^{rr}_{\mathit{eff}}$} & {$\Xi^{rr}_{\mathit{stick}}$} & {$\Xi^{rr}_{\mathit{eff}}$ / $\Xi^{rr}_{\mathit{stick}}$}\\ 
{} & {} & {\footnotesize(amu A$^2$ / fs)} & {\footnotesize(amu A$^2$ / fs)} & {\footnotesize(amu A$^2$ / fs)} & \\  \hline
Sphere (r = 20 \AA) & {$x = y = z$} & 0 & {2386} & {3314} & {0.720}\\
Sphere (r = 30 \AA) & {$x = y = z$} & 0 & {8415} & {11749} & {0.716}\\
Sphere (r = 40 \AA) & {$x = y = z$} & 0 & {47544} & {34464} & {1.380}\\ 
Prolate Ellipsoid & {$x = y$} & {1792} & {3128} & {4991} & {0.627}\\
Prolate Ellipsoid & {$z$} & {716} & {1590} & {1993} & {0.798}
\\ \hline \hline
\label{table:couple}
\end{longtable}

The results for $\Xi^{rr}_{\mathit{eff}}$ show that, contrary to the flat Au(111) / hexane interface, gold structures solvated by hexane do not exist in the ``slip'' boundary conditions. At this length scale, the nanostructures are not perfect spheroids due to atomic `roughening' of the surface and therefore experience increased interfacial friction which deviates from the ideal ``slip'' case. The 20 and 30 \AA$\,$ radius nanoparticles experience approximately 70\% of the ideal ``stick'' boundary interfacial friction. Rotation of the ellipsoid about its long axis more closely approaches the ``stick'' limit than rotation about the short axis, which may at first seem counterintuitive. However, the `propellor' motion caused by rotation about short axis may exclude solvent from the rotation cavity or move a sufficient amount of solvent along with the gold that a smaller interfacial friction is actually experienced. The largest nanoparticle (40 \AA$\,$ radius) appears to experience more than the ``stick'' limit of interfacial friction, which may be a consequence of surface features or anomalous solvent behaviors that are not fully understood at this time.

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% **DISCUSSION**
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\section{Discussion} 


%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% **ACKNOWLEDGMENTS**
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\section*{Acknowledgments} 

We gratefully acknowledge conversations with Dr. Shenyu Kuang. Support for
this project was provided by the National Science Foundation under grant
CHE-0848243. Computational time was provided by the Center for Research
Computing (CRC) at the University of Notre Dame.

\newpage

\bibliography{nonperiodicVSS}

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