--- trunk/tengDissertation/Langevin.tex 2006/07/17 15:48:57 2939 +++ trunk/tengDissertation/Langevin.tex 2006/07/18 15:23:09 2949 @@ -11,27 +11,27 @@ solvent simulations for dynamical properties\cite{Shen simulations. Implicit solvent Langevin dynamics simulations of met-enkephalin not only outperform explicit solvent simulations for computational efficiency, but also agrees very well with explicit -solvent simulations for dynamical properties\cite{Shen2002}. +solvent simulations for dynamical properties.\cite{Shen2002} Recently, applying Langevin dynamics with the UNRES model, Liow and his coworkers suggest that protein folding pathways can be possibly -explored within a reasonable amount of time\cite{Liwo2005}. The +explored within a reasonable amount of time.\cite{Liwo2005} The stochastic nature of the Langevin dynamics also enhances the sampling of the system and increases the probability of crossing -energy barriers\cite{Banerjee2004, Cui2003}. Combining Langevin +energy barriers.\cite{Banerjee2004, Cui2003} Combining Langevin dynamics with Kramers's theory, Klimov and Thirumalai identified free-energy barriers by studying the viscosity dependence of the -protein folding rates\cite{Klimov1997}. In order to account for +protein folding rates.\cite{Klimov1997} In order to account for solvent induced interactions missing from implicit solvent model, Kaya incorporated desolvation free energy barrier into implicit coarse-grained solvent model in protein folding/unfolding studies and discovered a higher free energy barrier between the native and denatured states. Because of its stability against noise, Langevin dynamics is very suitable for studying remagnetization processes in -various systems\cite{Palacios1998,Berkov2002,Denisov2003}. For +various systems.\cite{Palacios1998,Berkov2002,Denisov2003} For instance, the oscillation power spectrum of nanoparticles from Langevin dynamics simulation has the same peak frequencies for different wave vectors, which recovers the property of magnetic -excitations in small finite structures\cite{Berkov2005a}. +excitations in small finite structures.\cite{Berkov2005a} %review langevin/browninan dynamics for arbitrarily shaped rigid body Combining Langevin or Brownian dynamics with rigid body dynamics, @@ -40,14 +40,14 @@ torques\cite{Mielke2004}. Membrane fusion is another k as well as excluded volume potentials, Mielke and his coworkers discovered rapid superhelical stress generations from the stochastic simulation of twin supercoiling DNA with response to induced -torques\cite{Mielke2004}. Membrane fusion is another key biological +torques.\cite{Mielke2004} Membrane fusion is another key biological process which controls a variety of physiological functions, such as release of neurotransmitters \textit{etc}. A typical fusion event happens on the time scale of a millisecond, which is impractical to study using atomistic models with newtonian mechanics. With the help of coarse-grained rigid body model and stochastic dynamics, the fusion pathways were explored by many -researchers\cite{Noguchi2001,Noguchi2002,Shillcock2005}. Due to the +researchers.\cite{Noguchi2001,Noguchi2002,Shillcock2005} Due to the difficulty of numerical integration of anisotropic rotation, most of the rigid body models are simply modeled using spheres, cylinders, ellipsoids or other regular shapes in stochastic simulations. In an @@ -56,20 +56,20 @@ consecutive rotations\cite{Fernandes2002}. Unfortunate dynamics simulation algorithm\cite{Ermak1978,Allison1991} by incorporating a generalized $6\times6$ diffusion tensor and introducing a simple rotation evolution scheme consisting of three -consecutive rotations\cite{Fernandes2002}. Unfortunately, unexpected +consecutive rotations.\cite{Fernandes2002} Unfortunately, unexpected errors and biases are introduced into the system due to the arbitrary order of applying the noncommuting rotation -operators\cite{Beard2003}. Based on the observation the momentum +operators.\cite{Beard2003} Based on the observation the momentum relaxation time is much less than the time step, one may ignore the inertia in Brownian dynamics. However, the assumption of zero average acceleration is not always true for cooperative motion which is common in protein motion. An inertial Brownian dynamics (IBD) was proposed to address this issue by adding an inertial correction -term\cite{Beard2000}. As a complement to IBD which has a lower bound +term.\cite{Beard2000} As a complement to IBD which has a lower bound in time step because of the inertial relaxation time, long-time-step inertial dynamics (LTID) can be used to investigate the inertial behavior of the polymer segments in low friction -regime\cite{Beard2000}. LTID can also deal with the rotational +regime.\cite{Beard2000} LTID can also deal with the rotational dynamics for nonskew bodies without translation-rotation coupling by separating the translation and rotation motion and taking advantage of the analytical solution of hydrodynamics properties. However, @@ -223,7 +223,7 @@ Garc\'{i}a de la Torre and Bloomfield\cite{Torre1977}, Here $R_{ij}$ is the distance vector between bead $i$ and bead $j$. A second order expression for element of different size was introduced by Rotne and Prager\cite{Rotne1969} and improved by -Garc\'{i}a de la Torre and Bloomfield\cite{Torre1977}, +Garc\'{i}a de la Torre and Bloomfield,\cite{Torre1977} \begin{equation} T_{ij} = \frac{1}{{8\pi \eta R_{ij} }}\left[ {\left( {I + \frac{{R_{ij} R_{ij}^T }}{{R_{ij}^2 }}} \right) + R\frac{{\sigma @@ -576,17 +576,7 @@ -0.1988 $\rm{\AA}$), as well as the resistance tensor, 0.2057&0&0&0&3.219&10.7373\\ \end{array}} \right). \] -%\[ -%\left( {\begin{array}{*{20}c} -%0.9261 & 1.310e-14 & -7.292e-15&5.067e-14&0.08585&0.2057\\ -%3.968e-14& 0.9270&-0.007063& 0.08585&6.764e-14&4.846e-14\\ -%-6.561e-16&-0.007063&0.7494&0.2057&4.846e-14&1.5036e-14\\ -%5.067e-14&0.0858&0.2057& 58.64& 8.563e-13&-8.5736\\ -%0.08585&6.764e-14&4.846e-14&1.555e-12&48.30&3.219&\\ -%0.2057&4.846e-14&1.5036e-14&-3.904e-13&3.219&10.7373\\ -%\end{array}} \right). -%\] - +where the units for translational, translation-rotation coupling and rotational tensors are $\frac{kcal \cdot fs}{mol \cdot \rm{\AA}^2}$, $\frac{kcal \cdot fs}{mol \cdot \rm{\AA} \cdot rad}$ and $\frac{kcal \cdot fs}{mol \cdot rad^2}$ respectively. Curves of the velocity auto-correlation functions in Fig.~\ref{langevin:vacf} were shown to match each other very well. However, because of the stochastic nature, simulation using Langevin @@ -594,8 +584,7 @@ probably due to the reason that the viscosity using in study the rotational motion of the molecules, we also calculated the auto-correlation function of the principle axis of the second GB particle, $u$. The discrepancy shown in Fig.~\ref{langevin:uacf} was -probably due to the reason that the viscosity using in the -simulations only partially preserved the dynamics of the system. +probably due to the reason that we used the experimental viscosity directly instead of calculating bulk viscosity from simulation. \begin{figure} \centering