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\caption{Example analysis of the conformational dynamics of a pentapeptide backbone:
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(a)~The Trp-Leu-Ala-Leu-Leu pentapeptide in licorice representation~\cite{vmd}.
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(b)~The VAMP-2 score indicates which of the tested featurizations contains the highest kinetic variance.
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(c)~The sample free energy projected onto the first two time-lagged independent components (ICs) at lag time $\tau=0.5$~ns shows multiple minima and
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(c)~The sample free energy projected onto the first two time-lagged independent components (ICs) at lag time $\tau=\SI{0.5}{\nano\second}$ shows multiple minima and
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(d)~the time series of the first two ICs of the first trajectory show rare jumps.}
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.
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We show one example of this ranking and the absolute VAMP-2 scores for lag time~\SI{0.5}{\nano\second} in Fig.~\ref{fig:io-to-tica}b.
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We find that backbone torsions contain more kinetic variance than the backbone heavy atom positions or the distances between them (Fig.~\ref{fig:io-to-tica}b).
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This suggests that backbone torsions are the best of the options evaluated for MSM construction.
to a lower dimensional space that can be discretized with higher resolution and better statistical efficiency.
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TICA is a special case of the variational principle~\cite{noe-vac,nueske-vamk} and is designed to find a projection preserving the long-timescale dynamics in the dataset.
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Here, performing TICA on the backbone torsions at lag time~$0.5$~ns yields a four dimensional subspace using a~$95\%$ kinetic variance cutoff
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Here, performing TICA on the backbone torsions at lag time~\SI{0.5}{\nano\second} yields a four dimensional subspace using a~\SI{95}{\percent} kinetic variance cutoff
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(note that we perform a $\cos/\sin$-transformation of the torsions before TICA in order to preserve their periodicity).
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The sample free energy projected onto the first two independent components (ICs) exhibits several minima (Fig.~\ref{fig:io-to-tica}c).
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Discrete jumps between the minima can be observed by visualizing the transformation of the first trajectory into these ICs (Fig.~\ref{fig:io-to-tica}d).
@@ -435,19 +435,19 @@ \subsection{MSM estimation and validation}
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(a)~The convergence behavior of the implied timescales associated with the four slowest processes.
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The solid lines 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 (marking equality of timescale and lag time) with grey area indicates the timescale horizon below which the MSM cannot resolve processes.
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As implied timescales are well-converged at $\tau=0.5$~ns, this lag time is chosen for subsequent MSM estimation.
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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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As implied timescales are well-converged at $\tau=\SI{0.5}{\nano\second}$, this lag time is chosen for subsequent MSM estimation.
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(b)~Chapman-Kolmogorov test computed using an MSM estimated with lag time $\tau=\SI{0.5}{\nano\second}$ assuming~5 metastable states.
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Predictions from this model agree with higher lag time estimates within confidence intervals.
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Implied timescales convergence as well as a passing Chapman-Kolmogorov test are a necessary condition in MSM validation.
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In both panels, the (non-grey) shaded areas indicate~$95\%$ confidence intervals computed with the aforementioned Bayesian sampling procedure.}
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In both panels, the (non-grey) shaded areas indicate~\SI{95}{\percent} confidence intervals computed with the aforementioned Bayesian sampling procedure.}
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\label{fig:its-and-ck}
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\end{figure}
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A necessary condition for Markovian dynamics in our reduced space is that the ITS are approximately constant as a function of $\tau$;
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accordingly, we chose the smallest possible $\tau$ which fulfills this condition within the model uncertainty.
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The uncertainty bounds are computed using a Bayesian scheme~\cite{ben-rev-msm,noe-tmat-sampling} with~$100$ samples.
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In our example, we find that the four slowest ITS converge quickly and are constant within a $95\%$ confidence interval for lag times above~$0.5$~ns (Fig.~\ref{fig:its-and-ck}a).
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Using this lag time we can now estimate a (Bayesian) MSM with $\tau=0.5$~ns.
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In our example, we find that the four slowest ITS converge quickly and are constant within a \SI{95}{\percent} confidence interval for lag times above~\SI{0.5}{\nano\second} (Fig.~\ref{fig:its-and-ck}a).
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Using this lag time we can now estimate a (Bayesian) MSM with $\tau=\SI{0.5}{\nano\second}$.
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To test the validity of our MSM, we perform a Chapman-Kolmogorov (CK) test.
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Visualizing the full transition probability matrix $T$ is difficult;
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The CK test (Fig.~\ref{fig:its-and-ck}b) shows that predictions from our MSM (blue-dashed lines)
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agrees well with MSMs estimated with longer lag times (black-solid lines)
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Thus, the CK test confirms that five metastable states is an appropriate choice
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and shows that the MSM we have estimated at lag time $\tau=0.5$~ns indeed predicts the
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and shows that the MSM we have estimated at lag time $\tau=\SI{0.5}{\nano\second}$ indeed predicts the
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long-timescale behavior of our system within error (blue/shaded area).
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In notebook~03, we demonstrate in detail how to estimate and validate MSMs with PyEMMA.
@@ -525,7 +525,7 @@ \subsection{Analyzing the MSM}
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The mean first passage times (MFPTs) out of and into the macrostate $\mathcal{S}_1$ compute to
@@ -551,7 +551,7 @@ \subsection{Connecting the MSM with experimental data}
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(a)~the Trp-1 SASA autocorrelation function yields a weak signal which, however,
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(b)~can be enhanced if the system is prepared in the nonequilibrium condition $\mathcal{S}_1$.
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The solid/orange lines denote the maximum likelihood MSM result;
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the dashed/blue lines and the the shaded areas indicate sample means and~$95\%$ confidence intervals computed with a Bayesian sampling procedure~\cite{ben-rev-msm}.}
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the dashed/blue lines and the the shaded areas indicate sample means and~\SI{95}{\percent} confidence intervals computed with a Bayesian sampling procedure~\cite{ben-rev-msm}.}
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