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1   \chapter{\label{chapt:introduction}INTRODUCTION AND THEORETICAL BACKGROUND}
2  
3 \section{\label{introSection:molecularDynamics}Molecular Dynamics}
4
5 As a special discipline of molecular modeling, Molecular dynamics
6 has proven to be a powerful tool for studying the functions of
7 biological systems, providing structural, thermodynamic and
8 dynamical information.
9
3   \section{\label{introSection:classicalMechanics}Classical
4   Mechanics}
5  
# Line 22 | Line 15 | sufficient to predict the future behavior of the syste
15   sufficient to predict the future behavior of the system.
16  
17   \subsection{\label{introSection:newtonian}Newtonian Mechanics}
18 + The discovery of Newton's three laws of mechanics which govern the
19 + motion of particles is the foundation of the classical mechanics.
20 + Newton¡¯s first law defines a class of inertial frames. Inertial
21 + frames are reference frames where a particle not interacting with
22 + other bodies will move with constant speed in the same direction.
23 + With respect to inertial frames Newton¡¯s second law has the form
24 + \begin{equation}
25 + F = \frac {dp}{dt} = \frac {mv}{dt}
26 + \label{introEquation:newtonSecondLaw}
27 + \end{equation}
28 + A point mass interacting with other bodies moves with the
29 + acceleration along the direction of the force acting on it. Let
30 + $F_ij$ be the force that particle $i$ exerts on particle $j$, and
31 + $F_ji$ be the force that particle $j$ exerts on particle $i$.
32 + Newton¡¯s third law states that
33 + \begin{equation}
34 + F_ij = -F_ji
35 + \label{introEquation:newtonThirdLaw}
36 + \end{equation}
37  
38 + Conservation laws of Newtonian Mechanics play very important roles
39 + in solving mechanics problems. The linear momentum of a particle is
40 + conserved if it is free or it experiences no force. The second
41 + conservation theorem concerns the angular momentum of a particle.
42 + The angular momentum $L$ of a particle with respect to an origin
43 + from which $r$ is measured is defined to be
44 + \begin{equation}
45 + L \equiv r \times p \label{introEquation:angularMomentumDefinition}
46 + \end{equation}
47 + The torque $\tau$ with respect to the same origin is defined to be
48 + \begin{equation}
49 + N \equiv r \times F \label{introEquation:torqueDefinition}
50 + \end{equation}
51 + Differentiating Eq.~\ref{introEquation:angularMomentumDefinition},
52 + \[
53 + \dot L = \frac{d}{{dt}}(r \times p) = (\dot r \times p) + (r \times
54 + \dot p)
55 + \]
56 + since
57 + \[
58 + \dot r \times p = \dot r \times mv = m\dot r \times \dot r \equiv 0
59 + \]
60 + thus,
61 + \begin{equation}
62 + \dot L = r \times \dot p = N
63 + \end{equation}
64 + If there are no external torques acting on a body, the angular
65 + momentum of it is conserved. The last conservation theorem state
66 + that if all forces are conservative, Energy
67 + \begin{equation}E = T + V \label{introEquation:energyConservation}
68 + \end{equation}
69 + is conserved. All of these conserved quantities are
70 + important factors to determine the quality of numerical integration
71 + scheme for rigid body \cite{Dullweber1997}.
72 +
73   \subsection{\label{introSection:lagrangian}Lagrangian Mechanics}
74  
75   Newtonian Mechanics suffers from two important limitations: it
# Line 36 | Line 83 | system, alternative procedures may be developed.
83   which arise in attempts to apply Newton's equation to complex
84   system, alternative procedures may be developed.
85  
86 < \subsection{\label{introSection:halmiltonPrinciple}Hamilton's
86 > \subsubsection{\label{introSection:halmiltonPrinciple}Hamilton's
87   Principle}
88  
89   Hamilton introduced the dynamical principle upon which it is
# Line 46 | Line 93 | the kinetic, $K$, and potential energies, $U$.
93   The actual trajectory, along which a dynamical system may move from
94   one point to another within a specified time, is derived by finding
95   the path which minimizes the time integral of the difference between
96 < the kinetic, $K$, and potential energies, $U$.
96 > the kinetic, $K$, and potential energies, $U$ \cite{tolman79}.
97   \begin{equation}
98   \delta \int_{t_1 }^{t_2 } {(K - U)dt = 0} ,
99   \label{introEquation:halmitonianPrinciple1}
# Line 67 | Line 114 | then Eq.~\ref{introEquation:halmitonianPrinciple1} bec
114   \label{introEquation:halmitonianPrinciple2}
115   \end{equation}
116  
117 < \subsection{\label{introSection:equationOfMotionLagrangian}The
117 > \subsubsection{\label{introSection:equationOfMotionLagrangian}The
118   Equations of Motion in Lagrangian Mechanics}
119  
120   for a holonomic system of $f$ degrees of freedom, the equations of
# Line 142 | Line 189 | known as the canonical equations of motions.
189   Eq.~\ref{introEquation:motionHamiltonianCoordinate} and
190   Eq.~\ref{introEquation:motionHamiltonianMomentum} are Hamilton's
191   equation of motion. Due to their symmetrical formula, they are also
192 < known as the canonical equations of motions.
192 > known as the canonical equations of motions \cite{Goldstein01}.
193  
194   An important difference between Lagrangian approach and the
195   Hamiltonian approach is that the Lagrangian is considered to be a
# Line 153 | Line 200 | equations.
200   appropriate for application to statistical mechanics and quantum
201   mechanics, since it treats the coordinate and its time derivative as
202   independent variables and it only works with 1st-order differential
203 < equations.
203 > equations\cite{Marion90}.
204  
205 < \subsection{\label{introSection:poissonBrackets}Poisson Brackets}
205 > In Newtonian Mechanics, a system described by conservative forces
206 > conserves the total energy \ref{introEquation:energyConservation}.
207 > It follows that Hamilton's equations of motion conserve the total
208 > Hamiltonian.
209 > \begin{equation}
210 > \frac{{dH}}{{dt}} = \sum\limits_i {\left( {\frac{{\partial
211 > H}}{{\partial q_i }}\dot q_i  + \frac{{\partial H}}{{\partial p_i
212 > }}\dot p_i } \right)}  = \sum\limits_i {\left( {\frac{{\partial
213 > H}}{{\partial q_i }}\frac{{\partial H}}{{\partial p_i }} -
214 > \frac{{\partial H}}{{\partial p_i }}\frac{{\partial H}}{{\partial
215 > q_i }}} \right) = 0}
216 > \label{introEquation:conserveHalmitonian}
217 > \end{equation}
218  
219 < \subsection{\label{introSection:canonicalTransformation}Canonical
220 < Transformation}
219 > When studying Hamiltonian system, it is more convenient to use
220 > notation
221 > \begin{equation}
222 > r = r(q,p)^T
223 > \end{equation}
224 > and to introduce a $2n \times 2n$ canonical structure matrix $J$,
225 > \begin{equation}
226 > J = \left( {\begin{array}{*{20}c}
227 >   0 & I  \\
228 >   { - I} & 0  \\
229 > \end{array}} \right)
230 > \label{introEquation:canonicalMatrix}
231 > \end{equation}
232 > where $I$ is a $n \times n$ identity matrix and $J$ is a
233 > skew-symmetric matrix ($ J^T  =  - J $). Thus, Hamiltonian system
234 > can be rewritten as,
235 > \begin{equation}
236 > \frac{d}{{dt}}r = J\nabla _r H(r)
237 > \label{introEquation:compactHamiltonian}
238 > \end{equation}
239  
240   \section{\label{introSection:statisticalMechanics}Statistical
241   Mechanics}
242  
243 < The thermodynamic behaviors and properties  of Molecular Dynamics
243 > The thermodynamic behaviors and properties of Molecular Dynamics
244   simulation are governed by the principle of Statistical Mechanics.
245   The following section will give a brief introduction to some of the
246   Statistical Mechanics concepts presented in this dissertation.
247  
248 < \subsection{\label{introSection::ensemble}Ensemble}
248 > \subsection{\label{introSection:ensemble}Ensemble and Phase Space}
249  
250   \subsection{\label{introSection:ergodic}The Ergodic Hypothesis}
251  
252 + Various thermodynamic properties can be calculated from Molecular
253 + Dynamics simulation. By comparing experimental values with the
254 + calculated properties, one can determine the accuracy of the
255 + simulation and the quality of the underlying model. However, both of
256 + experiment and computer simulation are usually performed during a
257 + certain time interval and the measurements are averaged over a
258 + period of them which is different from the average behavior of
259 + many-body system in Statistical Mechanics. Fortunately, Ergodic
260 + Hypothesis is proposed to make a connection between time average and
261 + ensemble average. It states that time average and average over the
262 + statistical ensemble are identical \cite{Frenkel1996, leach01:mm}.
263 + \begin{equation}
264 + \langle A \rangle_t = \mathop {\lim }\limits_{t \to \infty }
265 + \frac{1}{t}\int\limits_0^t {A(p(t),q(t))dt = \int\limits_\Gamma
266 + {A(p(t),q(t))} } \rho (p(t), q(t)) dpdq
267 + \end{equation}
268 + where $\langle A \rangle_t$ is an equilibrium value of a physical
269 + quantity and $\rho (p(t), q(t))$ is the equilibrium distribution
270 + function. If an observation is averaged over a sufficiently long
271 + time (longer than relaxation time), all accessible microstates in
272 + phase space are assumed to be equally probed, giving a properly
273 + weighted statistical average. This allows the researcher freedom of
274 + choice when deciding how best to measure a given observable. In case
275 + an ensemble averaged approach sounds most reasonable, the Monte
276 + Carlo techniques\cite{metropolis:1949} can be utilized. Or if the
277 + system lends itself to a time averaging approach, the Molecular
278 + Dynamics techniques in Sec.~\ref{introSection:molecularDynamics}
279 + will be the best choice\cite{Frenkel1996}.
280 +
281 + \section{\label{introSection:geometricIntegratos}Geometric Integrators}
282 + A variety of numerical integrators were proposed to simulate the
283 + motions. They usually begin with an initial conditionals and move
284 + the objects in the direction governed by the differential equations.
285 + However, most of them ignore the hidden physical law contained
286 + within the equations. Since 1990, geometric integrators, which
287 + preserve various phase-flow invariants such as symplectic structure,
288 + volume and time reversal symmetry, are developed to address this
289 + issue. The velocity verlet method, which happens to be a simple
290 + example of symplectic integrator, continues to gain its popularity
291 + in molecular dynamics community. This fact can be partly explained
292 + by its geometric nature.
293 +
294 + \subsection{\label{introSection:symplecticManifold}Symplectic Manifold}
295 + A \emph{manifold} is an abstract mathematical space. It locally
296 + looks like Euclidean space, but when viewed globally, it may have
297 + more complicate structure. A good example of manifold is the surface
298 + of Earth. It seems to be flat locally, but it is round if viewed as
299 + a whole. A \emph{differentiable manifold} (also known as
300 + \emph{smooth manifold}) is a manifold with an open cover in which
301 + the covering neighborhoods are all smoothly isomorphic to one
302 + another. In other words,it is possible to apply calculus on
303 + \emph{differentiable manifold}. A \emph{symplectic manifold} is
304 + defined as a pair $(M, \omega)$ which consisting of a
305 + \emph{differentiable manifold} $M$ and a close, non-degenerated,
306 + bilinear symplectic form, $\omega$. A symplectic form on a vector
307 + space $V$ is a function $\omega(x, y)$ which satisfies
308 + $\omega(\lambda_1x_1+\lambda_2x_2, y) = \lambda_1\omega(x_1, y)+
309 + \lambda_2\omega(x_2, y)$, $\omega(x, y) = - \omega(y, x)$ and
310 + $\omega(x, x) = 0$. Cross product operation in vector field is an
311 + example of symplectic form.
312 +
313 + One of the motivations to study \emph{symplectic manifold} in
314 + Hamiltonian Mechanics is that a symplectic manifold can represent
315 + all possible configurations of the system and the phase space of the
316 + system can be described by it's cotangent bundle. Every symplectic
317 + manifold is even dimensional. For instance, in Hamilton equations,
318 + coordinate and momentum always appear in pairs.
319 +
320 + Let  $(M,\omega)$ and $(N, \eta)$ be symplectic manifolds. A map
321 + \[
322 + f : M \rightarrow N
323 + \]
324 + is a \emph{symplectomorphism} if it is a \emph{diffeomorphims} and
325 + the \emph{pullback} of $\eta$ under f is equal to $\omega$.
326 + Canonical transformation is an example of symplectomorphism in
327 + classical mechanics. According to Liouville's theorem, the
328 + Hamiltonian \emph{flow} or \emph{symplectomorphism} generated by the
329 + Hamiltonian vector filed preserves the volume form on the phase
330 + space, which is the basis of classical statistical mechanics.
331 +
332 + \subsection{\label{introSection:exactFlow}The Exact Flow of ODE}
333 +
334 + \subsection{\label{introSection:hamiltonianSplitting}Hamiltonian Splitting}
335 +
336 + \section{\label{introSection:molecularDynamics}Molecular Dynamics}
337 +
338 + As a special discipline of molecular modeling, Molecular dynamics
339 + has proven to be a powerful tool for studying the functions of
340 + biological systems, providing structural, thermodynamic and
341 + dynamical information.
342 +
343 + \subsection{\label{introSec:mdInit}Initialization}
344 +
345 + \subsection{\label{introSection:mdIntegration} Integration of the Equations of Motion}
346 +
347   \section{\label{introSection:rigidBody}Dynamics of Rigid Bodies}
348  
349 + A rigid body is a body in which the distance between any two given
350 + points of a rigid body remains constant regardless of external
351 + forces exerted on it. A rigid body therefore conserves its shape
352 + during its motion.
353 +
354 + Applications of dynamics of rigid bodies.
355 +
356 + \subsection{\label{introSection:lieAlgebra}Lie Algebra}
357 +
358 + \subsection{\label{introSection:DLMMotionEquation}The Euler Equations of Rigid Body Motion}
359 +
360 + \subsection{\label{introSection:otherRBMotionEquation}Other Formulations for Rigid Body Motion}
361 +
362 + %\subsection{\label{introSection:poissonBrackets}Poisson Brackets}
363 +
364   \section{\label{introSection:correlationFunctions}Correlation Functions}
365  
366   \section{\label{introSection:langevinDynamics}Langevin Dynamics}
367  
368 + \subsection{\label{introSection:LDIntroduction}Introduction and application of Langevin Dynamics}
369 +
370   \subsection{\label{introSection:generalizedLangevinDynamics}Generalized Langevin Dynamics}
371  
372 < \subsection{\label{introSection:hydroynamics}Hydrodynamics}
372 > \begin{equation}
373 > H = \frac{{p^2 }}{{2m}} + U(x) + H_B  + \Delta U(x,x_1 , \ldots x_N)
374 > \label{introEquation:bathGLE}
375 > \end{equation}
376 > where $H_B$ is harmonic bath Hamiltonian,
377 > \[
378 > H_B  =\sum\limits_{\alpha  = 1}^N {\left\{ {\frac{{p_\alpha ^2
379 > }}{{2m_\alpha  }} + \frac{1}{2}m_\alpha  w_\alpha ^2 } \right\}}
380 > \]
381 > and $\Delta U$ is bilinear system-bath coupling,
382 > \[
383 > \Delta U =  - \sum\limits_{\alpha  = 1}^N {g_\alpha  x_\alpha  x}
384 > \]
385 > Completing the square,
386 > \[
387 > H_B  + \Delta U = \sum\limits_{\alpha  = 1}^N {\left\{
388 > {\frac{{p_\alpha ^2 }}{{2m_\alpha  }} + \frac{1}{2}m_\alpha
389 > w_\alpha ^2 \left( {x_\alpha   - \frac{{g_\alpha  }}{{m_\alpha
390 > w_\alpha ^2 }}x} \right)^2 } \right\}}  - \sum\limits_{\alpha  =
391 > 1}^N {\frac{{g_\alpha ^2 }}{{2m_\alpha  w_\alpha ^2 }}} x^2
392 > \]
393 > and putting it back into Eq.~\ref{introEquation:bathGLE},
394 > \[
395 > H = \frac{{p^2 }}{{2m}} + W(x) + \sum\limits_{\alpha  = 1}^N
396 > {\left\{ {\frac{{p_\alpha ^2 }}{{2m_\alpha  }} + \frac{1}{2}m_\alpha
397 > w_\alpha ^2 \left( {x_\alpha   - \frac{{g_\alpha  }}{{m_\alpha
398 > w_\alpha ^2 }}x} \right)^2 } \right\}}
399 > \]
400 > where
401 > \[
402 > W(x) = U(x) - \sum\limits_{\alpha  = 1}^N {\frac{{g_\alpha ^2
403 > }}{{2m_\alpha  w_\alpha ^2 }}} x^2
404 > \]
405 > Since the first two terms of the new Hamiltonian depend only on the
406 > system coordinates, we can get the equations of motion for
407 > Generalized Langevin Dynamics by Hamilton's equations
408 > \ref{introEquation:motionHamiltonianCoordinate,
409 > introEquation:motionHamiltonianMomentum},
410 > \begin{align}
411 > \dot p &=  - \frac{{\partial H}}{{\partial x}}
412 >       &= m\ddot x
413 >       &= - \frac{{\partial W(x)}}{{\partial x}} - \sum\limits_{\alpha  = 1}^N {g_\alpha  \left( {x_\alpha   - \frac{{g_\alpha  }}{{m_\alpha  w_\alpha ^2 }}x} \right)}
414 > \label{introEq:Lp5}
415 > \end{align}
416 > , and
417 > \begin{align}
418 > \dot p_\alpha   &=  - \frac{{\partial H}}{{\partial x_\alpha  }}
419 >                &= m\ddot x_\alpha
420 >                &= \- m_\alpha  w_\alpha ^2 \left( {x_\alpha   - \frac{{g_\alpha}}{{m_\alpha  w_\alpha ^2 }}x} \right)
421 > \end{align}
422 >
423 > \subsection{\label{introSection:laplaceTransform}The Laplace Transform}
424 >
425 > \[
426 > L(x) = \int_0^\infty  {x(t)e^{ - pt} dt}
427 > \]
428 >
429 > \[
430 > L(x + y) = L(x) + L(y)
431 > \]
432 >
433 > \[
434 > L(ax) = aL(x)
435 > \]
436 >
437 > \[
438 > L(\dot x) = pL(x) - px(0)
439 > \]
440 >
441 > \[
442 > L(\ddot x) = p^2 L(x) - px(0) - \dot x(0)
443 > \]
444 >
445 > \[
446 > L\left( {\int_0^t {g(t - \tau )h(\tau )d\tau } } \right) = G(p)H(p)
447 > \]
448 >
449 > Some relatively important transformation,
450 > \[
451 > L(\cos at) = \frac{p}{{p^2  + a^2 }}
452 > \]
453 >
454 > \[
455 > L(\sin at) = \frac{a}{{p^2  + a^2 }}
456 > \]
457 >
458 > \[
459 > L(1) = \frac{1}{p}
460 > \]
461 >
462 > First, the bath coordinates,
463 > \[
464 > p^2 L(x_\alpha  ) - px_\alpha  (0) - \dot x_\alpha  (0) =  - \omega
465 > _\alpha ^2 L(x_\alpha  ) + \frac{{g_\alpha  }}{{\omega _\alpha
466 > }}L(x)
467 > \]
468 > \[
469 > L(x_\alpha  ) = \frac{{\frac{{g_\alpha  }}{{\omega _\alpha  }}L(x) +
470 > px_\alpha  (0) + \dot x_\alpha  (0)}}{{p^2  + \omega _\alpha ^2 }}
471 > \]
472 > Then, the system coordinates,
473 > \begin{align}
474 > mL(\ddot x) &=  - \frac{1}{p}\frac{{\partial W(x)}}{{\partial x}} -
475 > \sum\limits_{\alpha  = 1}^N {\left\{ {\frac{{\frac{{g_\alpha
476 > }}{{\omega _\alpha  }}L(x) + px_\alpha  (0) + \dot x_\alpha
477 > (0)}}{{p^2  + \omega _\alpha ^2 }} - \frac{{g_\alpha ^2 }}{{m_\alpha
478 > }}\omega _\alpha ^2 L(x)} \right\}}
479 > %
480 > &= - \frac{1}{p}\frac{{\partial W(x)}}{{\partial x}} -
481 > \sum\limits_{\alpha  = 1}^N {\left\{ { - \frac{{g_\alpha ^2 }}{{m_\alpha  \omega _\alpha ^2 }}\frac{p}{{p^2  + \omega _\alpha ^2 }}pL(x)
482 > - \frac{p}{{p^2  + \omega _\alpha ^2 }}g_\alpha  x_\alpha  (0)
483 > - \frac{1}{{p^2  + \omega _\alpha ^2 }}g_\alpha  \dot x_\alpha  (0)} \right\}}
484 > \end{align}
485 > Then, the inverse transform,
486 >
487 > \begin{align}
488 > m\ddot x &=  - \frac{{\partial W(x)}}{{\partial x}} -
489 > \sum\limits_{\alpha  = 1}^N {\left\{ {\left( { - \frac{{g_\alpha ^2
490 > }}{{m_\alpha  \omega _\alpha ^2 }}} \right)\int_0^t {\cos (\omega
491 > _\alpha  t)\dot x(t - \tau )d\tau  - \left[ {g_\alpha  x_\alpha  (0)
492 > - \frac{{g_\alpha  }}{{m_\alpha  \omega _\alpha  }}} \right]\cos
493 > (\omega _\alpha  t) - \frac{{g_\alpha  \dot x_\alpha  (0)}}{{\omega
494 > _\alpha  }}\sin (\omega _\alpha  t)} } \right\}}
495 > %
496 > &= - \frac{{\partial W(x)}}{{\partial x}} - \int_0^t
497 > {\sum\limits_{\alpha  = 1}^N {\left( { - \frac{{g_\alpha ^2
498 > }}{{m_\alpha  \omega _\alpha ^2 }}} \right)\cos (\omega _\alpha
499 > t)\dot x(t - \tau )d} \tau }  + \sum\limits_{\alpha  = 1}^N {\left\{
500 > {\left[ {g_\alpha  x_\alpha  (0) - \frac{{g_\alpha  }}{{m_\alpha
501 > \omega _\alpha  }}} \right]\cos (\omega _\alpha  t) +
502 > \frac{{g_\alpha  \dot x_\alpha  (0)}}{{\omega _\alpha  }}\sin
503 > (\omega _\alpha  t)} \right\}}
504 > \end{align}
505 >
506 > \begin{equation}
507 > m\ddot x =  - \frac{{\partial W}}{{\partial x}} - \int_0^t {\xi
508 > (t)\dot x(t - \tau )d\tau }  + R(t)
509 > \label{introEuqation:GeneralizedLangevinDynamics}
510 > \end{equation}
511 > %where $ {\xi (t)}$ is friction kernel, $R(t)$ is random force and
512 > %$W$ is the potential of mean force. $W(x) =  - kT\ln p(x)$
513 > \[
514 > \xi (t) = \sum\limits_{\alpha  = 1}^N {\left( { - \frac{{g_\alpha ^2
515 > }}{{m_\alpha  \omega _\alpha ^2 }}} \right)\cos (\omega _\alpha  t)}
516 > \]
517 > For an infinite harmonic bath, we can use the spectral density and
518 > an integral over frequencies.
519 >
520 > \[
521 > R(t) = \sum\limits_{\alpha  = 1}^N {\left( {g_\alpha  x_\alpha  (0)
522 > - \frac{{g_\alpha ^2 }}{{m_\alpha  \omega _\alpha ^2 }}x(0)}
523 > \right)\cos (\omega _\alpha  t)}  + \frac{{\dot x_\alpha
524 > (0)}}{{\omega _\alpha  }}\sin (\omega _\alpha  t)
525 > \]
526 > The random forces depend only on initial conditions.
527 >
528 > \subsubsection{\label{introSection:secondFluctuationDissipation}The Second Fluctuation Dissipation Theorem}
529 > So we can define a new set of coordinates,
530 > \[
531 > q_\alpha  (t) = x_\alpha  (t) - \frac{1}{{m_\alpha  \omega _\alpha
532 > ^2 }}x(0)
533 > \]
534 > This makes
535 > \[
536 > R(t) = \sum\limits_{\alpha  = 1}^N {g_\alpha  q_\alpha  (t)}
537 > \]
538 > And since the $q$ coordinates are harmonic oscillators,
539 > \[
540 > \begin{array}{l}
541 > \left\langle {q_\alpha  (t)q_\alpha  (0)} \right\rangle  = \left\langle {q_\alpha ^2 (0)} \right\rangle \cos (\omega _\alpha  t) \\
542 > \left\langle {q_\alpha  (t)q_\beta  (0)} \right\rangle  = \delta _{\alpha \beta } \left\langle {q_\alpha  (t)q_\alpha  (0)} \right\rangle  \\
543 > \end{array}
544 > \]
545 >
546 > \begin{align}
547 > \left\langle {R(t)R(0)} \right\rangle  &= \sum\limits_\alpha
548 > {\sum\limits_\beta  {g_\alpha  g_\beta  \left\langle {q_\alpha
549 > (t)q_\beta  (0)} \right\rangle } }
550 > %
551 > &= \sum\limits_\alpha  {g_\alpha ^2 \left\langle {q_\alpha ^2 (0)}
552 > \right\rangle \cos (\omega _\alpha  t)}
553 > %
554 > &= kT\xi (t)
555 > \end{align}
556 >
557 > \begin{equation}
558 > \xi (t) = \left\langle {R(t)R(0)} \right\rangle
559 > \label{introEquation:secondFluctuationDissipation}
560 > \end{equation}
561 >
562 > \section{\label{introSection:hydroynamics}Hydrodynamics}
563 >
564 > \subsection{\label{introSection:frictionTensor} Friction Tensor}
565 > \subsection{\label{introSection:analyticalApproach}Analytical
566 > Approach}
567 >
568 > \subsection{\label{introSection:approximationApproach}Approximation
569 > Approach}
570 >
571 > \subsection{\label{introSection:centersRigidBody}Centers of Rigid
572 > Body}

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