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Copy file name to clipboardExpand all lines: manuscript/manuscript.tex
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@@ -152,7 +152,7 @@ \subsection{Essential theory}
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The highest eigenvalue, $\lambda_1(\tau)$, is unique and is equal to $1$, and its corresponding left eigenvector $\phi_1$ corresponds to the stationary distribution, $\pi$.
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From the relationship between the left and right eigenvectors (eq.~\ref{eq:left-right-eigenvalue-relation}) we see that the right eigenvector $\psi_1$ is a vector consisting of~$1$'s.
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The subsequent eigenvalues $\lambda_{i>1}(\tau)$ are real with absolute values less than~$1$ and are related to the \emph{characteristic} or \emph{implied} time-scales of dynamical processes within the system (eq.~\ref{eq:its}).
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The subsequent eigenvalues $\lambda_{i>1}(\tau)$ are real with absolute values less than~$1$ and are related to the \emph{characteristic} or \emph{implied} timescales of dynamical processes within the system (eq.~\ref{eq:its}).
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The right eigenvectors $\psi_i$ each encode a dynamical process (for $i>1$), corresponding to the characteristic time-scale, $t_i$.
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The coefficients of the eigenvectors represent the flux into and out of the Markov states that characterizes that process.
\caption{Example analysis of the conformational dynamics of a pentapeptide backbone:
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(a)~The convergence behavior of the implied timescales associated with the four slowest processes.
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(b)~Chapman-Kolmogorov test computed using an MSM estimated with lag time $\tau=0.5$~ns assuming~5 meta-stable states.
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(b)~Chapman-Kolmogorov test computed using an MSM estimated with lag time $\tau=0.5$~ns assuming~5 metastable states.
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The solid lines in (a) refer to the maximum likelihood result while the dashed lines show the ensemble mean computed with a Bayesian sampling procedure~\cite{ben-rev-msm}.
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The black line indicates where implied timescales are equal to the lag time, whereas the grey area indicates all implied timescales faster than the lag time.
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In both panels, the (non-grey) shaded areas indicate~$95\%$ confidence intervals computed with the aforementioned Bayesian sampling procedure.}
Here, we utilize the VAMP-2 score, which maximizes the kinetic variance contained in the features~\cite{kinetic-maps}.
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We should always evaluate the score in a cross-validated manner to ensure that we neither include too few features (under-fitting) or too many features (over-fitting)~\cite{gmrq,vamp-preprint}.
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To choose among three different molecular features reflecting protein structure, we compute the (cross-validated) VAMP-2 score (Notebook 00).
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Although we cannot optimize lag times with a variational score, such as VAMP-2, it is important to ensure that properties that we optimize are robust as a function of lag time.
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Although we cannot MSM optimize lag times with a variational score\cite{husic2017note}, such as VAMP-2, it is important to ensure that properties that we optimize are robust as a function of lag time.
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Consequently, we compute the VAMP-2 score at several lag times (Notebook 00).
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We find that the relative rankings of the different molecular features are highly robust as a function of lag time.
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We show one example of this ranking and the absolute VAMP-2 scores for lag time~$0.5$~ns in Fig.~\ref{fig:io-to-tica}b.
@@ -362,10 +362,10 @@ \subsection{Analyzing the MSM}
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where $\pi_j$ denotes the MSM stationary weight of the $j^\textrm{th}$ microstate.
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In order to interpret the slowest relaxation timescales, we refer to the (right) eigenvectors of the MSM.
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We here consider the right eigenvectors as they are normalized by the stationary distribution of the MSM.
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This enables us to dissect dynamic changes with finite time-scales from the equilbrium distribution of states.
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We here chose the right eigenvectors as they are independent of the stationary distribution of the MSM.
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This enables us to specifically study what conformational changes are happening on a particular time scale independently of the equilbrium distribution.
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The first right eigenvector corresponds to the stationary process and its eigenvalue is the Perron eigenvalue~$1$.
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The second right eigenvector, on the other hand, corresponds to the slowest process.
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The second right eigenvector, on the other hand, corresponds to the slowest process in the system.
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Note that the eigenvectors are real as detailed balance has been enforced during MSM estimation.
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The minimal and maximal components of the second right eigenvector indicate the microstates between which the process shifts probability density.
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The relaxation timescale of this exchange process is exactly the corresponding implied timescale, which can be computed from its corresponding eigenvalue using~\eqref{eq:its}.
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\subsection{Advanced Methods}
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The present tutorial presents the basics of modern Markov state modelling with PyEMMA.
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However, recent years have seen many extensions of the methodology -- many of which are available within PyEMMA.
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We encourage interested readers to look into these methods in the software documentation and to check out the Jupyter notebooks distributed with PyEMMA.
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The present tutorial presents the basics of modern Markov state modeling with PyEMMA.
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However, recent years have seen many extensions of the methodology --- many of which are available within PyEMMA.
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We encourage interested readers to look into these methods in the software documentation and to make use of the specific Jupyter notebooks distributed with PyEMMA.
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Conventional Markov state modelling often relies on large simulation datasets to ensure proper convergence of thermodynamic and kinetic properties.
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Conventional Markov state modeling often relies on large simulation datasets to ensure proper convergence of thermodynamic and kinetic properties.
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In one extension, Multi-ensemble Markov models (MEMMs)~\cite{dtram,tram}, we can integrate unbiased and biased simulations in a systematic manner to speed up the convergence.
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MEMMs consequently enable users to combine enhanced sampling methods such as umbrella sampling or replica exchange with conventional molecular dynamics simulations to more efficiently study rare event kinetics~\cite{trammbar}.
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MEMMs are implemented in PyEMMA.
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Another issue often faced during Markov state modelling is a lack of quantitative agreement with complementary experimental data.
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This issue is not intrinsic to the Markov state modelling approach as such, but rather associated with systematic errors in the force field model used to conduct the simulation.
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Another issue often faced during Markov state modeling is a lack of quantitative agreement with complementary experimental data.
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This issue is not intrinsic to the Markov state modeling approach as such, but rather associated with systematic errors in the force field model used to conduct the simulation.
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Nevertheless, using Augmented Markov models (AMM) it is possible to build an integrative MSM which balances experimental and simulation data, taking into account their respective uncertainties~\cite{simon-amm}.
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