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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 path which minimizes the time integral of the diff
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 | equation of motion. Due to their symmetrical formula,
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 | independent variables and it only works with 1st-order
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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