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\begin{document} |
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|
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\title{Supplemental Material for: Real space electrostatics for |
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multipoles. III. Dielectric Properties} |
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|
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\author{Madan Lamichhane} |
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\affiliation{Department of Physics, University |
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of Notre Dame, Notre Dame, IN 46556} |
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\author{Thomas Parsons} |
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\affiliation{Department of Chemistry and Biochemistry, University |
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of Notre Dame, Notre Dame, IN 46556} |
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\author{Kathie E. Newman} |
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\affiliation{Department of Physics, University |
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of Notre Dame, Notre Dame, IN 46556} |
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\author{J. Daniel Gezelter} |
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\email{gezelter@nd.edu.} |
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\affiliation{Department of Chemistry and Biochemistry, University |
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of Notre Dame, Notre Dame, IN 46556} |
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|
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\date{\today}% It is always \today, today, |
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% but any date may be explicitly specified |
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|
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\begin{abstract} |
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This document includes useful relationships for computing the |
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interactions between fields and field gradients and point multipolar |
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representations of molecular electrostatics. We also provide |
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explanatory derivations of a number of relationships used in the |
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main text. This includes the Boltzmann averages of quadrupole |
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orientations, and the interaction of a quadrupole density with the |
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self-generated field gradient. This last relationship is assumed to |
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be zero in the main text but is explicitly shown to be zero here. |
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\end{abstract} |
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|
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\maketitle |
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|
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\section{Generating Uniform Field Gradients} |
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One important task in carrying out the simulations mentioned in the |
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main text was to generate uniform electric field gradients. To do |
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this, we relied heavily on both the notation and results from Torres |
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del Castillo and Mend\'{e}z Garido.\cite{Torres-del-Castillo:2006uo} |
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In this work, tensors were expressed in Cartesian components, using at |
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times a dyadic notation. This proves quite useful for computer |
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simulations that make use of toroidal boundary conditions. |
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|
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An alternative formalism uses the theory of angular momentum and |
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spherical harmonics and is common in standard physics texts such as |
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Jackson,\cite{Jackson98} Morse and Feshbach,\cite{Morse:1946zr} and |
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Stone.\cite{Stone:1997ly} Because this approach has its own |
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advantages, relationships are provided below comparing that |
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terminology to the Cartesian tensor notation. |
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|
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The gradient of the electric field, |
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\begin{equation*} |
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\mathsf{G}(\mathbf{r}) = -\nabla \nabla \Phi(\mathbf{r}), |
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\end{equation*} |
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where $\Phi(\mathbf{r})$ is the electrostatic potential. In a |
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charge-free region of space, $\nabla \cdot \mathbf{E}=0$, and |
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$\mathsf{G}$ is a symmetric traceless tensor. From symmetry |
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arguments, we know that this tensor can be written in terms of just |
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five independent components. |
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|
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Following Torres del Castillo and Mend\'{e}z Garido's notation, the |
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gradient of the electric field may also be written in terms of two |
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vectors $\mathbf{a}$ and $\mathbf{b}$, |
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\begin{equation*} |
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G_{ij}=\frac{1}{2} (a_i b_j + a_j b_i) - \frac{1}{3}(\mathbf a \cdot \mathbf b) \delta_{ij} . |
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\end{equation*} |
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If the vectors $\mathbf{a}$ and $\mathbf{b}$ are unit vectors, the |
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electrostatic potential that generates a uniform gradient may be |
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written: |
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\begin{align} |
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\Phi(x, y, z) =\; -\frac{g_o}{2} & \left(\left(a_1b_1 - |
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\frac{cos\psi}{3}\right)\;x^2+\left(a_2b_2 |
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- \frac{cos\psi}{3}\right)\;y^2 + |
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\left(a_3b_3 - |
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\frac{cos\psi}{3}\right)\;z^2 \right. \nonumber \\ |
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& + (a_1b_2 + a_2b_1)\; xy + (a_1b_3 + a_3b_1)\; xz + (a_2b_3 + a_3b_2)\; yz \bigg) . |
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\label{eq:appliedPotential} |
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\end{align} |
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Note $\mathbf{a}\cdot\mathbf{a} = \mathbf{b} \cdot \mathbf{b} = 1$, |
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$\mathbf{a} \cdot \mathbf{b}=\cos \psi$, and $g_0$ is the overall |
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strength of the potential. |
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|
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Taking the gradient of Eq. (\ref{eq:appliedPotential}), we find the |
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field due to this potential, |
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\begin{equation} |
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\mathbf{E} = -\nabla \Phi |
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=\frac{g_o}{2} \left(\begin{array}{ccc} |
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2(a_1 b_1 - \frac{cos\psi}{3})\; x & +\; (a_1 b_2 + a_2 b_1)\; y & +\; (a_1 b_3 + a_3 b_1)\; z \\ |
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(a_2 b_1 + a_1 b_2)\; x & +\; 2(a_2 b_2 - \frac{cos\psi}{3})\; y & +\; (a_2 b_3 + a_3 b_3)\; z \\ |
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(a_3 b_1 + a_3 b_2)\; x & +\; (a_3 b_2 + a_2 b_3)\; y & +\; 2(a_3 b_3 - \frac{cos\psi}{3})\; z |
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\end{array} \right), |
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\label{eq:CE} |
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\end{equation} |
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while the gradient of the electric field in this form, |
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\begin{equation} |
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\mathsf{G} = \nabla\mathbf{E} |
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= \frac{g_o}{2}\left(\begin{array}{ccc} |
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2(a_1\; b_1 - \frac{cos\psi}{3}) & (a_1\; b_2 \;+ a_2\; b_1) & (a_1\; b_3 \;+ a_3\; b_1) \\ |
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(a_2\; b_1 \;+ a_1\; b_2) & 2(a_2\; b_2 \;- \frac{cos\psi}{3}) & (a_2\; b_3 \;+ a_3\; b_3) \\ |
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(a_3\; b_1 \;+ a_3\; b_2) & (a_3\; b_2 \;+ a_2\; b_3) & 2(a_3\; b_3 \;- \frac{cos\psi}{3}) |
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\end{array} \right), |
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\label{eq:GC} |
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\end{equation} |
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is uniform over the entire space. Therefore, to describe a uniform |
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gradient in this notation, two unit vectors ($\mathbf{a}$ and |
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$\mathbf{b}$) as well as a potential strength, $g_0$, must be |
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specified. As expected, this requires five independent parameters. |
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|
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The common alternative to the Cartesian notation expresses the |
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electrostatic potential using the notation of Morse and |
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Feshbach,\cite{Morse:1946zr} |
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\begin{equation} \label{eq:quad_phi} |
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\Phi(x,y,z) = -\left[ a_{20} \frac{2 z^2 -x^2 - y^2}{2} |
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+ 3 a_{21}^e \,xz + 3 a_{21}^o \,yz |
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+ 6a_{22}^e \,xy + 3 a_{22}^o (x^2 - y^2) \right]. |
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\end{equation} |
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Here we use the standard $(l,m)$ form for the $a_{lm}$ coefficients, |
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with superscript $e$ and $o$ denoting even and odd, respectively. |
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This form makes the functional analogy to ``d'' atomic states |
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apparent. |
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|
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Applying the gradient operator to Eq. (\ref{eq:quad_phi}) the electric |
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field due to this potential, |
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\begin{equation} |
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\mathbf{E} = -\nabla \Phi |
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= \left(\begin{array}{ccc} |
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\left( 6a_{22}^o -a_{20} \right)\; x &+\; 6a_{22}^e\; y &+\; 3a_{21}^e\; z \\ |
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6a_{22}^e\; x & -\; (a_{20} + 6a_{22}^o)\; y & +\; 3a_{21}^o\; z \\ |
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3a_{21}^e\; x & +\; 3a_{21}^o\; y & +\; 2a_{20}\; z |
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\end{array} \right), |
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\label{eq:MFE} |
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\end{equation} |
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while the gradient of the electric field in this form is: |
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\begin{equation} \label{eq:grad_e2} |
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\mathsf{G} = |
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\begin{pmatrix} |
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6 a_{22}^o - a_{20} & 6a_{22}^e & 3a_{21}^e\\ |
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6a_{22}^e & -(a_{20}+6a_{22}^o) & 3a_{21}^o \\ |
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3a_{21}^e & 3a_{21}^o & 2a_{20} \\ |
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\end{pmatrix} \\ |
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\end{equation} |
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which is also uniform over the entire space. This form for the |
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gradient can be factored as |
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\begin{gather} |
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\begin{aligned} |
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\mathsf{G} = a_{20} |
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\begin{pmatrix} |
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-1 & 0 & 0\\ |
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0 & -1 & 0\\ |
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0 & 0 & 2\\ |
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\end{pmatrix} |
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+3a_{21}^e |
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\begin{pmatrix} |
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0 & 0 & 1\\ |
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0 & 0 & 0\\ |
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1 & 0 & 0\\ |
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\end{pmatrix} |
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+3a_{21}^o |
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\begin{pmatrix} |
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0 & 0 & 0\\ |
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0 & 0 & 1\\ |
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0 & 1 & 0\\ |
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\end{pmatrix} |
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+6a_{22}^e |
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\begin{pmatrix} |
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0 & 1 & 0\\ |
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1 & 0 & 0\\ |
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0 & 0 & 0\\ |
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\end{pmatrix} |
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+6a_{22}^o |
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\begin{pmatrix} |
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1 & 0 & 0\\ |
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0 & -1 & 0\\ |
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0 & 0 & 0\\ |
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\end{pmatrix} |
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\end{aligned} |
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\label{eq:intro_tensors} |
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\end{gather} |
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The five matrices in the expression above represent five different |
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symmetric traceless tensors of rank 2. |
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|
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It is useful to find the Cartesian vectors $\mathbf a$ and $\mathbf b$ |
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that generate the five types of tensors shown in |
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Eq. (\ref{eq:intro_tensors}). If the two vectors are co-linear, e.g., |
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$\psi=0$, $\mathbf{a}=(0,0,1)$ and $\mathbf{b}=(0,0,1)$, then |
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\begin{equation*} |
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\mathsf{G} = \frac{g_0}{3} |
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\begin{pmatrix} |
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-1 & 0 & 0 \\ |
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0 & -1 & 0 \\ |
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0 & 0 & 2 \\ |
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\end{pmatrix} , |
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\end{equation*} |
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which is the $a_{20}$ symmetry. |
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To generate the $a_{22}^o$ symmetry, we take: |
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$\mathbf{a}= (\frac{1}{\sqrt{2}}, \frac{1}{\sqrt{2}},0)$ and |
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$\mathbf{b}=(\frac{1}{\sqrt{2}}, -\frac{1}{\sqrt{2}},0)$ |
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and find: |
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\begin{equation*} |
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\mathsf{G}=\frac{g_0}{2} |
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\begin{pmatrix} |
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1 & 0 & 0 \\ |
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0 & -1 & 0 \\ |
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0 & 0 & 0 \\ |
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\end{pmatrix} . |
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\end{equation*} |
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To generate the $a_{22}^e$ symmetry, we take: |
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$\mathbf{a}= (1, 0, 0)$ and $\mathbf{b} = (0,1,0)$ and find: |
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\begin{equation*} |
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\mathsf{G}=\frac{g_0}{2} |
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\begin{pmatrix} |
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0 & 1 & 0 \\ |
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1 & 0 & 0 \\ |
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0 & 0 & 0 \\ |
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\end{pmatrix} . |
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\end{equation*} |
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The pattern is straightforward to continue for the other symmetries. |
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|
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We find the notation of Ref. \onlinecite{Torres-del-Castillo:2006uo} |
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helpful when creating specific types of constant gradient electric |
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fields in simulations. For this reason, |
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Eqs. (\ref{eq:appliedPotential}), (\ref{eq:GC}), and (\ref{eq:CE}) are |
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implemented in our code. In the simulations using constant applied |
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gradients that are mentioned in the main text, we utilized a field |
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with the $a_{22}^e$ symmetry using vectors, $\mathbf{a}= (1, 0, 0)$ |
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and $\mathbf{b} = (0,1,0)$. |
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|
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\section{Point-multipolar interactions with a spatially-varying electric field} |
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|
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This section develops formulas for the force and torque exerted by an |
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external electric field, $\mathbf{E}(\mathbf{r})$, on object |
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$a$. Object $a$ has an embedded collection of charges and in |
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simulations will represent a molecule, ion, or a coarse-grained |
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substructure. We describe the charge distributions using primitive |
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multipoles defined in Ref. \onlinecite{PaperI} by |
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\begin{align} |
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C_a =&\sum_{k \, \text{in }a} q_k , \label{eq:charge} \\ |
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D_{a\alpha} =&\sum_{k \, \text{in }a} q_k r_{k\alpha}, \label{eq:dipole}\\ |
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Q_{a\alpha\beta} =& \frac{1}{2} \sum_{k \, \text{in } a} q_k |
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r_{k\alpha} r_{k\beta}, |
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\label{eq:quadrupole} |
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\end{align} |
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where $\mathbf{r}_k$ is the local coordinate system for the object |
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(usually the center of mass of object $a$). Components of vectors and |
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tensors are given using the Einstein repeated summation notation. Note |
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that the definition of the primitive quadrupole here differs from the |
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standard traceless form, and contains an additional Taylor-series |
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based factor of $1/2$. In Ref. \onlinecite{PaperI}, we derived the |
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forces and torques each object exerts on the other objects in the |
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system. |
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|
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Here we must also consider an external electric field that varies in |
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space: $\mathbf E(\mathbf r)$. Each of the local charges $q_k$ in |
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object $a$ will then experience a slightly different field. This |
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electric field can be expanded in a Taylor series around the local |
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origin of each object. For a particular charge $q_k$, the electric |
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field at that site's position is given by: |
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\begin{equation} |
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\mathbf{E}(\mathbf{r}_k) = E_\gamma|_{\mathbf{r}_k = 0} + \nabla_\delta E_\gamma |_{\mathbf{r}_k = 0} r_{k \delta} |
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+ \frac {1}{2} \nabla_\delta \nabla_\varepsilon E_\gamma|_{\mathbf{r}_k = 0} r_{k \delta} |
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r_{k \varepsilon} + ... |
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\end{equation} |
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Note that if one shrinks object $a$ to a single point, the |
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${E}_\gamma$ terms are all evaluated at the center of the object (now |
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a point). Thus later the ${E}_\gamma$ terms can be written using the |
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same (molecular) origin for all point charges in the object. The force |
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exerted on object $a$ by the electric field is given by,\cite{Raab:2004ve} |
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\begin{align} |
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F^a_\gamma = \sum_{k \textrm{~in~} a} E_\gamma(\mathbf{r}_k) &= \sum_{k \textrm{~in~} a} q_k \lbrace E_\gamma + \nabla_\delta E_\gamma r_{k \delta} |
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+ \frac {1}{2} \nabla_\delta \nabla_\varepsilon E_\gamma r_{k \delta} |
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r_{k \varepsilon} + ... \rbrace \\ |
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&= C_a E_\gamma + D_{a \delta} \nabla_\delta E_\gamma |
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+ Q_{a \delta \varepsilon} \nabla_\delta \nabla_\varepsilon E_\gamma + |
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... |
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\end{align} |
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Thus in terms of the global origin $\mathbf{r}$, ${F}_\gamma(\mathbf{r}) = C {E}_\gamma(\mathbf{r})$ etc. |
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|
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Similarly, the torque exerted by the field on $a$ can be expressed as |
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\begin{align} |
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\tau^a_\alpha &= \sum_{k \textrm{~in~} a} (\mathbf r_k \times q_k \mathbf E)_\alpha \\ |
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& = \sum_{k \textrm{~in~} a} \epsilon_{\alpha \beta \gamma} q_k |
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r_{k\beta} E_\gamma(\mathbf r_k) \\ |
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& = \epsilon_{\alpha \beta \gamma} D_\beta E_\gamma |
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+ 2 \epsilon_{\alpha \beta \gamma} Q_{\beta \delta} \nabla_\delta |
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E_\gamma + ... |
335 |
\end{align} |
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We note that the Levi-Civita symbol can be eliminated by utilizing the matrix cross product as defined in Ref. \onlinecite{Smith98}: |
337 |
\begin{equation} |
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\left[\mathsf{A} \times \mathsf{B}\right]_\alpha = \sum_\beta |
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\left[\mathsf{A}_{\alpha+1,\beta} \mathsf{B}_{\alpha+2,\beta} |
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-\mathsf{A}_{\alpha+2,\beta} \mathsf{B}_{\alpha+1,\beta} |
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\right] |
342 |
\label{eq:matrixCross} |
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\end{equation} |
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where $\alpha+1$ and $\alpha+2$ are regarded as cyclic permuations of |
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the matrix indices. Finally, the interaction energy $U^a$ of object $a$ with the external field is given by, |
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\begin{equation} |
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U^a = \sum_{k~in~a} q_k \phi_k (\mathrm{r}_k) |
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\end{equation} |
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Performing another Taylor series expansion about the local body origin, |
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\begin{equation} |
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\phi({\mathbf{r}_k}) = \phi|_{\mathbf{r}_k = 0 } + r_{k \alpha} \nabla_\alpha \phi_\alpha|_{\mathbf{r}_k = 0 } + \frac{1}{2} r_{k\alpha}r_{k\beta}\nabla_\alpha \nabla_\beta \phi|_{\mathbf{r}_k = 0} + ... |
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\end{equation} |
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Writing this in terms of the global origin $\mathrm{r}$, we find |
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\begin{equation} |
355 |
U(\mathbf{r}) = \mathrm{C} \phi(\mathbf{r}) - \mathrm{D}_\alpha \mathrm{E}_\alpha - \mathrm{Q}_{\alpha\beta}\nabla_\alpha \mathrm{E}_\beta + ... |
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\end{equation} |
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These results have been summarized in Table \ref{tab:UFT}. |
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|
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\begin{table} |
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\caption{Potential energy $(U)$, force $(\mathbf{F})$, and torque |
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$(\mathbf{\tau})$ expressions for a multipolar site at $\mathbf{r}$ in an |
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electric field, $\mathbf{E}(\mathbf{r})$ using the definitions of the multipoles in Eqs. (\ref{eq:charge}), (\ref{eq:dipole}) and (\ref{eq:quadrupole}). |
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\label{tab:UFT}} |
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\begin{tabular}{r|C{3cm}C{3cm}C{3cm}} |
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& Charge & Dipole & Quadrupole \\ \hline |
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$U(\mathbf{r})$ & $C \phi(\mathbf{r})$ & $-\mathbf{D} \cdot \mathbf{E}(\mathbf{r})$ & $- \mathsf{Q}:\nabla \mathbf{E}(\mathbf{r})$ \\ |
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$\mathbf{F}(\mathbf{r})$ & $C \mathbf{E}(\mathbf{r})$ & $\mathbf{D} \cdot \nabla \mathbf{E}(\mathbf{r})$ & $\mathsf{Q} : \nabla\nabla\mathbf{E}(\mathbf{r})$ \\ |
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$\mathbf{\tau}(\mathbf{r})$ & & $\mathbf{D} \times \mathbf{E}(\mathbf{r})$ & $2 \mathsf{Q} \times \nabla \mathbf{E}(\mathbf{r})$ |
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\end{tabular} |
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\end{table} |
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|
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\section{Boltzmann averages for orientational polarization} |
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If we consider a collection of molecules in the presence of external |
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field, the perturbation experienced by any one molecule will include |
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contributions to the field or field gradient produced by the all other |
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molecules in the system. In subsections |
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\ref{subsec:boltzAverage-Dipole} and \ref{subsec:boltzAverage-Quad}, |
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we discuss the molecular polarization due solely to external field |
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perturbations. This illustrates the origins of the polarizability |
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equations (Eqs. 6, 20, and 21) in the main text. |
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|
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\subsection{Dipoles} |
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\label{subsec:boltzAverage-Dipole} |
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Consider a system of molecules, each with permanent dipole moment |
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$p_o$. In the absense of an external field, thermal agitation orients |
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the dipoles randomly, and the system moment, $\mathbf{P}$, is zero. |
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External fields will line up the dipoles in the direction of applied |
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field. Here we consider the net field from all other molecules to be |
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zero. Therefore the total Hamiltonian acting on each molecule |
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is,\cite{Jackson98} |
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\begin{equation} |
392 |
H = H_o - \mathbf{p}_o \cdot \mathbf{E}, |
393 |
\end{equation} |
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where $H_o$ is a function of the internal coordinates of the molecule. |
395 |
The Boltzmann average of the dipole moment in the direction of the |
396 |
field is given by, |
397 |
\begin{equation} |
398 |
\langle p_{mol} \rangle = \frac{\displaystyle\int p_o \cos\theta |
399 |
e^{~p_o E \cos\theta /k_B T}\; d\Omega}{\displaystyle\int e^{~p_o E \cos\theta/k_B |
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T}\; d\Omega}, |
401 |
\end{equation} |
402 |
where the $z$-axis is taken in the direction of the applied field, |
403 |
$\bf{E}$ and |
404 |
$\int d\Omega = \int_0^\pi \sin\theta\; d\theta \int_0^{2\pi} d\phi |
405 |
\int_0^{2\pi} d\psi$ |
406 |
is an integration over Euler angles describing the orientation of the |
407 |
molecule. |
408 |
|
409 |
If the external fields are small, \textit{i.e.} |
410 |
$p_oE \cos\theta / k_B T << 1$, |
411 |
\begin{equation} |
412 |
\langle p_{mol} \rangle \approx \frac{{p_o}^2}{3 k_B T}E, |
413 |
\end{equation} |
414 |
where $ \alpha_p = \frac{{p_o}^2}{3 k_B T}$ is the molecular |
415 |
polarizability. The orientational polarization depends inversely on |
416 |
the temperature as the applied field must overcome thermal agitation |
417 |
to orient the dipoles. |
418 |
|
419 |
\subsection{Quadrupoles} |
420 |
\label{subsec:boltzAverage-Quad} |
421 |
If instead, our system consists of molecules with permanent |
422 |
\textit{quadrupole} tensor $q_{\alpha\beta}$. The average quadrupole |
423 |
at temperature $T$ in the presence of uniform applied field gradient |
424 |
is given by,\cite{AduGyamfi78, AduGyamfi81} |
425 |
\begin{equation} |
426 |
\langle q_{\alpha\beta} \rangle \;=\; \frac{\displaystyle\int |
427 |
q_{\alpha\beta}\; e^{-H/k_B T}\; d\Omega}{\displaystyle\int |
428 |
e^{-H/k_B T}\; d\Omega} \;=\; \frac{\displaystyle\int |
429 |
q_{\alpha\beta}\; e^{~q_{\mu\nu}\;\partial_\nu E_\mu /k_B T}\; |
430 |
d\Omega}{\displaystyle\int e^{~q_{\mu\nu}\;\partial_\nu E_\mu /k_B |
431 |
T}\; d\Omega }, |
432 |
\label{boltzQuad} |
433 |
\end{equation} |
434 |
where $H = H_o - q_{\mu\nu}\;\partial_\nu E_\mu $ is the energy of a |
435 |
quadrupole in the gradient of the applied field and $H_o$ is a |
436 |
function of internal coordinates of the molecule. The energy and |
437 |
quadrupole moment can be transformed into the body frame using a |
438 |
rotation matrix $\mathsf{\eta}^{-1}$, |
439 |
\begin{align} |
440 |
q_{\alpha\beta} &= \eta_{\alpha\alpha'}\;\eta_{\beta\beta'}\;{q}^* _{\alpha'\beta'} \\ |
441 |
H &= H_o - q:{\nabla}\mathbf{E} \\ |
442 |
&= H_o - q_{\mu\nu}\;\partial_\nu E_\mu \\ |
443 |
&= H_o |
444 |
-\eta_{\mu\mu'}\;\eta_{\nu\nu'}\;{q}^*_{\mu'\nu'}\;\partial_\nu |
445 |
E_\mu. \label{energyQuad} |
446 |
\end{align} |
447 |
Here the starred tensors are the components in the body fixed |
448 |
frame. Substituting equation (\ref{energyQuad}) in the equation |
449 |
(\ref{boltzQuad}) and taking linear terms in the expansion we obtain, |
450 |
\begin{equation} |
451 |
\braket{q_{\alpha\beta}} = \frac{\displaystyle \int q_{\alpha\beta} \left(1 + |
452 |
\frac{\eta_{\mu\mu'}\;\eta_{\nu\nu'}\;{q}^*_{\mu'\nu'}\;\partial_\nu |
453 |
E_\mu }{k_B T}\right)\; d\Omega}{\displaystyle \int \left(1 + \frac{\eta_{\mu\mu'}\;\eta_{\nu\nu'}\;{q}^*_{\mu'\nu'}\;\partial_\nu E_\mu }{k_B T}\right)\; d\Omega}. |
454 |
\end{equation} |
455 |
Recall that $\eta_{\alpha\alpha'}$ is the inverse of the rotation |
456 |
matrix that transforms the body fixed co-ordinates to the space |
457 |
co-ordinates. |
458 |
% \[\eta_{\alpha\alpha'} |
459 |
% = \left(\begin{array}{ccc} |
460 |
% cos\phi\; cos\psi - cos\theta\; sin\phi\; sin\psi & -cos\theta\; cos\psi\; sin\phi - cos\phi\; sin\psi & sin\theta\; sin\phi \\ |
461 |
% cos\psi\; sin\phi + cos\theta\; cos\phi \; sin\psi & cos\theta\; cos\phi\; cos\psi - sin\phi\; sin\psi & -cos\phi\; sin\theta \\ |
462 |
% sin\theta\; sin\psi & -cos\psi\; sin\theta & cos\theta |
463 |
% \end{array} \right).\] |
464 |
|
465 |
Integration of the first and second terms in the denominator gives |
466 |
$8 \pi^2$ and |
467 |
$8 \pi^2 ({\nabla} \cdot \mathbf{E}) \mathrm{Tr}(q^*) / 3 $ |
468 |
respectively. The second term vanishes for charge free space (where |
469 |
${\nabla} \cdot \mathbf{E}=0$). Similarly, integration of the first |
470 |
term in the numerator produces |
471 |
$8 \pi^2 \delta_{\alpha\beta} \mathrm{Tr}(q^*) / 3$ while the second |
472 |
produces |
473 |
$8 \pi^2 (3{q}^*_{\alpha'\beta'}{q}^*_{\beta'\alpha'} - |
474 |
{q}^*_{\alpha'\alpha'}{q}^*_{\beta'\beta'})\partial_\alpha E_\beta / |
475 |
15 k_B T $. |
476 |
Therefore the Boltzmann average of a quadrupole moment can be written |
477 |
as, |
478 |
\begin{equation} |
479 |
\langle q_{\alpha\beta} \rangle = \frac{1}{3} \mathrm{Tr}(q^*)\;\delta_{\alpha\beta} + \frac{{\bar{q_o}}^2}{15k_BT}\;\partial_\alpha E_\beta, |
480 |
\end{equation} |
481 |
where $\alpha_q = \frac{{\bar{q_o}}^2}{15k_BT} $ is a molecular |
482 |
quadrupole polarizablity and |
483 |
${\bar{q_o}}^2= |
484 |
3{q}^*_{\alpha'\beta'}{q}^*_{\beta'\alpha'}-{q}^*_{\alpha'\alpha'}{q}^*_{\beta'\beta'}$ |
485 |
is the square of the net quadrupole moment of a molecule. |
486 |
|
487 |
\section{Gradient of the field due to quadrupolar polarization} |
488 |
\label{singularQuad} |
489 |
In section IV.C of the main text, we stated that for quadrupolar |
490 |
fluids, the self-contribution to the field gradient vanishes at the |
491 |
singularity. In this section, we prove this statement. For this |
492 |
purpose, we consider a distribution of charge $\rho(\mathbf{r})$ which |
493 |
gives rise to an electric field $\mathbf{E}(\mathbf{r})$ and gradient |
494 |
of the field $\nabla\mathbf{E}(\mathbf{r})$ throughout space. The |
495 |
gradient of the electric field over volume due to the charges within |
496 |
the sphere of radius $R$ is given by (cf. Ref. \onlinecite{Jackson98}, |
497 |
equation 4.14): |
498 |
\begin{equation} |
499 |
\int_{r<R} \nabla\mathbf{E} d\mathbf{r} = -\int_{r=R} R^2 \mathbf{E}\;\hat{n}\; d\Omega |
500 |
\label{eq:8} |
501 |
\end{equation} |
502 |
where $d\Omega$ is the solid angle and $\hat{n}$ is the normal vector |
503 |
of the surface of the sphere, |
504 |
\begin{equation} |
505 |
\hat{n} = \sin\theta\cos\phi\; \hat{x} + \sin\theta\sin\phi\; \hat{y} + |
506 |
\cos\theta\; \hat{z} |
507 |
\end{equation} |
508 |
in spherical coordinates. For the charge density $\rho(\mathbf{r}')$, the |
509 |
total gradient of the electric field can be written as,\cite{Jackson98} |
510 |
\begin{equation} |
511 |
\int_{r<R} {\nabla}\mathbf {E}\; d\mathbf{r}=-\int_{r=R} R^2\; {\nabla}\Phi\; \hat{n}\; d\Omega =-\frac{1}{4\pi\;\epsilon_o}\int_{r=R} R^2\; {\nabla}\;\left(\int \frac{\rho(\mathbf r')}{|\mathbf{r}-\mathbf{r}'|}\;d\mathbf{r}'\right) \hat{n}\; d\Omega |
512 |
\label{eq:9} |
513 |
\end{equation} |
514 |
The radial function in the equation (\ref{eq:9}) can be expressed in |
515 |
terms of spherical harmonics as,\cite{Jackson98} |
516 |
\begin{equation} |
517 |
\frac{1}{|\mathbf{r} - \mathbf{r}'|} = 4\pi \sum_{l=0}^{\infty}\sum_{m=-l}^{m=l}\frac{1}{2l+1}\;\frac{{r^l_<}}{{r^{l+1}_>}}\;{Y^*}_{lm}(\theta', \phi')\;Y_{lm}(\theta, \phi) |
518 |
\label{eq:10} |
519 |
\end{equation} |
520 |
If the sphere completely encloses the charge density then $ r_< = r'$ and $r_> = R$. Substituting equation (\ref{eq:10}) into (\ref{eq:9}) we get, |
521 |
\begin{equation} |
522 |
\begin{split} |
523 |
\int_{r<R} {\nabla}\mathbf{E}\;d\mathbf{r} &=-\frac{R^2}{\epsilon_o}\int_{r=R} \; {\nabla}\;\left(\int \rho(\mathbf r')\sum_{l=0}^{\infty}\sum_{m=-l}^{m=l}\frac{1}{2l+1}\;\frac{{r'^l}}{{R^{l+1}}}\;{Y^*}_{lm}(\theta', \phi')\;Y_{lm}(\theta, \phi)\;d\mathbf{r}'\right) \hat{n}\; d\Omega \\ |
524 |
&= -\frac{R^2}{\epsilon_o}\sum_{l=0}^{\infty}\sum_{m=-l}^{m=l}\frac{1}{2l+1}\;\int \rho(\mathbf r')\;{r'^l}\;{Y^*}_{lm}(\theta', \phi')\left(\int_{r=R}\vec{\nabla}\left({R^{-(l+1)}}\;Y_{lm}(\theta, \phi)\right)\hat{n}\; d\Omega \right)d\mathbf{r} |
525 |
' |
526 |
\end{split} |
527 |
\label{eq:11} |
528 |
\end{equation} |
529 |
The gradient of the product of radial function and spherical harmonics |
530 |
is given by:\cite{Arfkan} |
531 |
\begin{equation} |
532 |
\begin{split} |
533 |
{\nabla}\left[ f(r)\;Y_{lm}(\theta, \phi)\right] = &-\left(\frac{l+1}{2l+1}\right)^{1/2}\; \left[\frac{\partial}{\partial r}-\frac{l}{r} \right]f(r)\; Y_{l, l+1, m}(\theta, \phi)\\ &+ \left(\frac{l}{2l+1}\right)^{1/2}\left[\frac |
534 |
{\partial}{\partial r}+\frac{l}{r} \right]f(r)\; Y_{l, l-1, m}(\theta, \phi). |
535 |
\end{split} |
536 |
\label{eq:12} |
537 |
\end{equation} |
538 |
where $Y_{l,l+1,m}(\theta, \phi)$ is a vector spherical |
539 |
harmonic.\cite{Arfkan} Using equation (\ref{eq:12}) we get, |
540 |
\begin{equation} |
541 |
{\nabla}\left({R^{-(l+1)}}\;Y_{lm}(\theta, \phi)\right) = [(l+1)(2l+1)]^{1/2}\; Y_{l,l+1,m}(\theta, \phi) \; \frac{1}{R^{l+2}}, |
542 |
\label{eq:13} |
543 |
\end{equation} |
544 |
Using Clebsch-Gordan coefficients $C(l+1,1,l|m_1,m_2,m)$, the vector |
545 |
spherical harmonics can be written in terms of spherical harmonics, |
546 |
\begin{equation} |
547 |
Y_{l,l+1,m}(\theta, \phi) = \sum_{m_1, m_2} C(l+1,1,l|m_1,m_2,m)\; Y_{l+1}^{m_1}(\theta,\phi)\; \hat{e}_{m_2}. |
548 |
\label{eq:14} |
549 |
\end{equation} |
550 |
Here $\hat{e}_{m_2}$ is a spherical tensor of rank 1 which can be expressed |
551 |
in terms of Cartesian coordinates, |
552 |
\begin{equation} |
553 |
{\hat{e}}_{+1} = - \frac{\hat{x}+i\hat{y}}{\sqrt{2}},\quad {\hat{e}}_{0} = \hat{z},\quad and \quad {\hat{e}}_{-1} = \frac{\hat{x}-i\hat{y}}{\sqrt{2}}. |
554 |
\label{eq:15} |
555 |
\end{equation} |
556 |
The normal vector $\hat{n} $ is then expressed in terms of spherical tensor of rank 1 as shown in below, |
557 |
\begin{equation} |
558 |
\hat{n} = \sqrt{\frac{4\pi}{3}}\left(-Y_1^{-1}{\hat{e}}_1 - Y_1^{1}{\hat{e}}_{-1} + Y_1^{0}{\hat{e}}_0 \right). |
559 |
\label{eq:16} |
560 |
\end{equation} |
561 |
The surface integral of the product of $\hat{n}$ and |
562 |
$Y_{l+1}^{m_1}(\theta, \phi)$ gives, |
563 |
\begin{equation} |
564 |
\begin{split} |
565 |
\int \hat{n}\;Y_{l+1}^{m_1}\;d\Omega &= \int \sqrt{\frac{4\pi}{3}}\left(-Y_1^{-1}{\hat{e}}_1 -Y_1^{1}{\hat{e}}_{-1} + Y_1^{0}{\hat{e}}_0 \right)\;Y_{l+1}^{m_1}\; d\Omega \\ |
566 |
&= \int \sqrt{\frac{4\pi}{3}}\left({Y_1^{1}}^* {\hat{e}}_1 +{Y_1^{-1}}^* {\hat{e}}_{-1} + {Y_1^{0}}^* {\hat{e}}_0 \right)\;Y_{l+1}^{m_1}\; d\Omega \\ |
567 |
&= \sqrt{\frac{4\pi}{3}}\left({\delta}_{l+1, 1}\;{\delta}_{1, m_1}\;{\hat{e}}_1 + {\delta}_{l+1, 1}\;{\delta}_{-1, m_1}\;{\hat{e}}_{-1}+ {\delta}_{l+1, 1}\;{\delta}_{0, m_1} \;{\hat{e}}_0\right), |
568 |
\end{split} |
569 |
\label{eq:17} |
570 |
\end{equation} |
571 |
where $Y_{l}^{-m} = (-1)^m\;{Y_{l}^{m}}^* $ and |
572 |
$ \int {Y_{l}^{m}}^* Y_{l'}^{m'}\;d\Omega = |
573 |
\delta_{ll'}\delta_{mm'} $. |
574 |
Non-vanishing values of equation \ref{eq:17} require $l = 0$, |
575 |
therefore the value of $ m = 0 $. Since the values of $ m_1$ are -1, |
576 |
1, and 0 then $m_2$ takes the values 1, -1, and 0, respectively |
577 |
provided that $m = m_1 + m_2$. Equation \ref{eq:11} can therefore be |
578 |
modified, |
579 |
\begin{equation} |
580 |
\begin{split} |
581 |
\int_{r<R} {\nabla}\mathbf{E}\;d\mathbf{r} = &- \sqrt{\frac{4\pi}{{3}}}\;\frac{1}{\epsilon_o}\int \rho(r')\;{Y^*}_{00}(\theta', \phi')[ C(1, 1, 0|-1,1,0)\;{\hat{e}_{-1}}{\hat{e}_{1}}\\ &+ C(1, 1, 0|-1,1,0)\;{\hat{e}_{1}}{\hat{e}_{-1}}+C( |
582 |
1, 1, 0|0,0,0)\;{\hat{e}_{0}}{\hat{e}_{0}} ]\; d\mathbf{r}' \\ |
583 |
&= -\sqrt{\frac{4\pi}{{3}}}\;\frac{1}{\epsilon_o}\int \rho(r')\;d\mathbf{r}'\left({\hat{e}_{-1}}{\hat{e}_{1}}+{\hat{e}_{1}}{\hat{e}_{-1}}-{\hat{e}_{0}}{\hat{e}_{0}}\right)\\ |
584 |
&= - \sqrt{\frac{4\pi}{{3}}}\;\frac{1}{\epsilon_o}\;C_\mathrm{total}\;\left({\hat{e}_{-1}}{\hat{e}_{1}}+{\hat{e}_{1}}{\hat{e}_{-1}}-{\hat{e}_{0}}{\hat{e}_{0}}\right). |
585 |
\end{split} |
586 |
\label{eq:19} |
587 |
\end{equation} |
588 |
In the last step, the charge density was integrated over the sphere, |
589 |
yielding a total charge $C_\mathrm{total}$.Equation (\ref{eq:19}) |
590 |
gives the total gradient of the field over a sphere due to the |
591 |
distribution of the charges. For quadrupolar fluids the total charge |
592 |
within a sphere is zero, therefore |
593 |
$ \int_{r<R} {\nabla}\mathbf{E}\;d\mathbf{r} = 0 $. Hence the quadrupolar |
594 |
polarization produces zero net gradient of the field inside the |
595 |
sphere. |
596 |
|
597 |
\bibliography{dielectric_new} |
598 |
\end{document} |
599 |
% |
600 |
% ****** End of file multipole.tex ****** |