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@@ -133,9 +133,9 @@ \subsection{Markov state models}
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For MD simulations in equilibrium, $P$ should obey detailed balance which is enforced by constraining the estimation of $P$ to the following equations:
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\begin{equation}
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\label{eq:balance}
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\pi_i P_{ij} = \pi_j P_{ji},
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\pi_i p_{ij} = \pi_j p_{ji},
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\end{equation}
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where $\pi_i$ is the stationary probability of state $i$ and $P_{ij}$ is the probability of transitioning to state $j$ conditional on being in state $i$.
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where $\pi_i$ is the stationary probability of state $i$ and $p_{ij}$ is the probability of transitioning to state $j$ conditional on being in state $i$.
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The constraints (\ref{eq:balance}) are omitted if MD simulations are not conducted in equilibrium, e.g.,
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for systems experiencing a pulling force or an external potential---see~\cite{Koltai2018} for a recent review on nonequilibrium MSMs.
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For the remainder of this section we will simplify the matter by assuming the more common scenario of MD simulations without external forces and (\ref{eq:balance}) to hold.
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Once we have used the ITS to choose the lag time, we can check whether a given transition probability matrix $T(\tau)$ is approximately Markovian using the Chapman-Kolmogorov (CK) test~\cite{noe-folding-pathways}.
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The CK property for a Markovian matrix is,
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\begin{equation}
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T(k \tau) = T^k(\tau),
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P(k \tau) = P^k(\tau),
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\end{equation}
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where the left-hand side of the equation corresponds to an MSM estimated at lag time $k\tau$, where $k$ is an integer larger than~1, whereas the right-hand side of the equation is our estimated MSM transition probability matrix to the $k^\textrm{th}$ power.
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where the left-hand side of the equation corresponds to an MSM estimated at lag time $k\tau$, where $k$ is an integer larger than~$1$, whereas the right-hand side of the equation is our estimated MSM transition probability matrix to the $k^\textrm{th}$ power.
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By assessing how well the approximated transition probability matrix adheres to the CK property, we can validate the appropriateness of the Markovian assumption for the model.
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Once validated, the transition matrix can be decomposed into eigenvectors and eigenvalues.
Note that the modeler has to select hyper-parameters at most stages throughout the workflow.
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This selection must be done carefully as poor choices make it hard, or even impossible, to build a good MSM.
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While there exist automated schemes~\cite{husic-optimized} for cross-validated optimization in the full hyper-parameter
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space, we chose to adopt a sequential approach where only the hyper-parameters of the current stage are optimized. This
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approach is not only computationally cheaper but allows us to discuss the significance of the necessary modeling
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choices.
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While there exist automated schemes~\cite{husic-optimized} for cross-validated optimization in the full hyper-parameter space,
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we chose to adopt a sequential approach where only the hyper-parameters of the current stage are optimized.
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This approach is not only computationally cheaper but allows us to discuss the significance of the necessary modeling choices.
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\subsection{Feature selection}
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\begin{figure}
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\includegraphics{figure_2}
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\caption{Example analysis of the conformational dynamics of a pentapeptide backbone: (a)~The Trp-Leu-Ala-Leu-Leu pentapeptide in licorice representation~\cite{vmd}.
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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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(d)~the time series of the first two ICs of the first trajectory show rare jumps.}
@@ -500,7 +500,7 @@ \subsection{Modeling large systems}
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Additional technical challenges for large systems include high demands on memory and computation time;
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we explain how to deal with those in the tutorials.
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More details on how to model complex systems with the techniques presented here are described e.g.~by~\cite{plattner_protein_2015,plattner_complete_2017}.
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More details on how to model complex systems with the techniques presented here are described, e.g., by~\cite{plattner_protein_2015,plattner_complete_2017}.
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