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g.tex
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g.tex
@ -189,7 +189,7 @@ In their pioneering work, \cite{Kalos_1974} Kalos and collaborators introduced t
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The domain used was the Cartesian product of small spheres around each particle, the Hamiltonian being approximated by the kinetic part only within the domain.
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Some time later, Kalos proposed to extend these ideas to more general domains such as rectangular and/or cylindrical domains. \cite{Kalos_2000} In both works, the size of the domains is infinitely small in the limit of a vanishing time step.
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Here, the domains are of arbitrary size, thus greatly increasing the efficiency of the approach.
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Note also that some general equations for arbitrary domains in continuous space have also been proposed by \titou{some} of us in Ref.~\onlinecite{Assaraf_1999B}.
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Note also that some general equations for arbitrary domains in continuous space have also been proposed by some of us in Ref.~\onlinecite{Assaraf_1999B}.
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Finally, from a general perspective, it is interesting to mention that the method proposed here is an illustration of how valuable and efficient can be the combination of stochastic and deterministic techniques.
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In recent years, a number of works have exploited this idea and proposed hybrid stochastic/deterministic schemes.
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@ -273,14 +273,12 @@ This is the central theme of the present work.
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%%% FIG 1 %%%
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\begin{figure*}
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\includegraphics[width=0.7\textwidth]{fig1}
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\label{fig:paths}
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\caption{
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\titou{$\Psi_0$ or $\Phi_0$?}
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Path integral representation of the exact coefficient $c_i=\braket{i}{\Psi_0}$ of the ground-state wave function $\ket{\Psi_0}$ obtained as an infinite sum of paths starting from $\ket{i_0}$ and ending at $\ket{i}$ [see Eq.\eqref{eq:G}].
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Path integral representation of the exact coefficient $c_i=\braket{i}{\Phi_0}$ of the ground-state wave function $\ket{\Phi_0}$ obtained as an infinite sum of paths starting from $\ket{i_0}$ and ending at $\ket{i}$ [see Eq.\eqref{eq:G}].
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Each path carries a weight $\prod_k T_{i_{k} i_{k+1}}$ computed along it.
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The result is independent of the choice of the initial state $\ket{i_0}$, provided that $\braket{i_0}{\Psi_0} \neq 0$.
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Here, only four paths of infinite length have been represented.
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}
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The result is independent of the choice of the initial state $\ket{i_0}$, provided that $\braket{i_0}{\Phi_0} \neq 0$.
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Here, only four paths of infinite length have been represented.}
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\label{fig:paths}
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\end{figure*}
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@ -422,7 +420,7 @@ To calculate the probabilistic averages, an artificial (mathematical) ``particle
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During the Monte Carlo simulation, the walker moves in configuration space by drawing new states with
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probability $p_{i_k \to i_{k+1}}$, thus realizing the path of probability $\text{Prob}_{i_0}$.
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Note that, instead of using a single walker, it is common to introduce a population of independent walkers and to calculate the averages over this population.
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In addition, thanks to the ergodic property of the stochastic matrix (see, for example, Ref.~\onlinecite{Caffarel_1988}), a unique path of infinite length from which sub-paths of length $N$ can be extracted may also be used.
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In addition, thanks to the ergodicity property of the stochastic matrix (see, for example, Ref.~\onlinecite{Caffarel_1988}), a unique path of infinite length from which sub-paths of length $N$ can be extracted may also be used.
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We shall not insist here on these practical details that are discussed, for example, in Refs.~\onlinecite{Foulkes_2001,Kolorenc_2011}.
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%{\it Spawner representation} In this representation, we no longer consider moving particles but occupied or non-occupied states $|i\rangle$.
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@ -501,7 +499,7 @@ Let us write an arbitrary path of length $N$ as
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\be
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\ket{i_0} \to \ket{i_1} \to \cdots \to \ket{i_N},
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\ee
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where the successive states are drawn using the transition probability matrix, $p_{i \to j}$.
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where the successive states are drawn using the transition probability matrix $p_{i \to j}$.
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This series can be recast
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\be
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\label{eq:eff_series}
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@ -510,19 +508,17 @@ This series can be recast
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where $\ket{I_0}=\ket{i_0}$ is the initial state, $n_0$ is the number of times the walker remains in the domain of $\ket{i_0}$ (with $1 \le n_0 \le N+1$), $\ket{I_1}$ is the first exit state that does not belong to $\cD_{i_0}$, $n_1$ is the number of times the walker remains in $\cD_{i_1}$ (with $1 \le n_1 \le N+1-n_0$), $\ket{I_2}$ is the second exit state, and so on.
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Here, the integer $p$ (with $0 \le p \le N$) indicates the number of exit events occurring along the path.
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The two extreme values, $p=0$ and $p=N$, correspond to the cases where the walker remains in the initial domain during the entire path, and where the walker exits a domain at each step, respectively.
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In what follows, we shall systematically write the integers representing the exit states in capital letter, while small letters will be used for
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denoting the elementary states $\ket{i_k}$ generated with $p_{i \to j}$. Making this distinction is important since the effective
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stochastic dynamics used in practical Monte Carlo calculations will only involve exit states, the contribution of the elementary states, $\ket{i_k}$, being
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exactly integrated out.
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Figure \ref{fig2} exemplifies how a path can be decomposed as proposed in Eq.(\ref{eq:eff_series}).
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To make things as clear as possible, let us explicit in detail how the path drawn in Figure \ref{fig2} evolves in time.
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The walker realizing the path starts at $\ket{i_0}$ within the domain $\cD_{i_0}$. It then makes two steps to arrive at $\ket{i_1}$, then $\ket{i_2}$ and, finally, leaves the domain
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at $\ket{i_3}$. The state $\ket{i_3}$ is the first exit state and is denoted as $\ket{I_1}(=\ket{i_3})$ following our convention of denoting exit states
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with capital letters. The trapping time in $\cD_{i_0}$ is $n_0=3$ since three states of the domain have been visited
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(namely, $\ket{i_0}$,$\ket{i_1}$,and $\ket{i_2}$).
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In what follows, we shall systematically label exit states with upper-case letters, while lower-case letters denote elementary states $\ket{i_k}$.
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Making this distinction is important since the effective stochastic dynamics used in practical Monte Carlo calculations only involve exit states $\ket{I_k}$, the contribution from the elementary states $\ket{i_k}$ being exactly integrated out.
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\titou{Figure \ref{fig:domains} exemplifies how a path can be decomposed as proposed in Eq.~\eqref{eq:eff_series}.
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To make things as clear as possible, let us explicit in detail how the path drawn in Fig.~\ref{fig:domains} evolves in time.
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The walker realizing the path starts at $\ket{i_0}$ within the domain $\cD_{i_0}$. It then makes two steps to arrive at $\ket{i_1}$, then $\ket{i_2}$ and, finally, leaves the domain at $\ket{i_3}$.
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The state $\ket{i_3}$ is the first exit state and is denoted as $\ket{I_1}(=\ket{i_3})$ following our convention of denoting exit states with capital letters.
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The trapping time in $\cD_{i_0}$ is $n_0=3$ since three states of the domain have been visited (namely, $\ket{i_0}$,$\ket{i_1}$,and $\ket{i_2}$).
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During the next steps the domains $\cD_{I_1}$, $\cD_{I_2}$, and $\cD_{I_3}$ are successively visited with $n_1=2$, $n_2=3$, and $n_3=1$, respectively.
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The corresponding exit states are $\ket{I_2}=\ket{i_5}$, $\ket{I_3}=\ket{i_8}$, and $\ket{I_4}=\ket{i_9}$, respectively. This work takes advantage of
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the fact that each possible path can be decomposed in this way.
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The corresponding exit states are $\ket{I_2}=\ket{i_5}$, $\ket{I_3}=\ket{i_8}$, and $\ket{I_4}=\ket{i_9}$, respectively.
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This work takes advantage of the fact that each possible path can be decomposed in this way.}
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%Generalizing what has been done for domains consisting of only one single state, the general idea here is to integrate out exactly the stochastic dynamics over the
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%set of all paths having the same representation, Eq.(\ref{eff_series}). As a consequence, an effective Monte Carlo dynamics including only exit states
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@ -531,15 +527,16 @@ the fact that each possible path can be decomposed in this way.
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%%% FIG 0B %%%
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\begin{figure}
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\includegraphics[width=\columnwidth,angle=0]{fig2}
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\caption{Representation of a path in terms of exit states, $\ket{I_k}$ and trapping times, $\ket{n_k}$. The
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\caption{Representation of a path in terms of exit states $\ket{I_k}$ and trapping times $\ket{n_k}$. The
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states $\ket{i_k}$ along the path are represented by small black circles and the exit states, $\ket{I_k}$, by larger black squares.
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By convention, the initial state is denoted using a capital letter, \ie, $\ket{i_0} = \ket{I_0}$, since it is the first state of the effective dynamics involving only exit states.
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See text for additional comments on the time evolution of the path.}
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\label{fig2}
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\label{fig:domains}
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\end{figure}
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Now, generalizing what has been done previously for a single-state domain, let us define the probability of remaining $n$ times in the domain of $\ket{I_0}$ and to exit at $\ket{I} \notin \cD_{I_0}$. This probability is given by
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Generalizing the single-state case treated previously, let us define the probability of remaining $n$ times in the domain of $\ket{I_0}$ and to exit at $\ket{I} \notin \cD_{I_0}$.
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This probability is given by
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\be
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\label{eq:eq1C}
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\cP_{I_0 \to I}(n)
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@ -556,7 +553,7 @@ the projection of the operator $T^+$ over the domain, \ie,
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\be
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T^+_I= P_I T^+ P_I.
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\ee
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Th operator $T^+_I$ governs the dynamics of the walkers trapped within the domain $\cD_{I}$,
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Th operator $T^+_I$ governs the dynamics of the walkers trapped in the domain $\cD_{I}$,
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see Eq.(\ref{eq:pij}) where $T^+$ is now restricted to the domain.
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Using Eqs.~\eqref{eq:pij} and \eqref{eq:eq1C}, the probability can be rewritten as
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\be
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