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@ -299,7 +299,7 @@ to be extrapolated. This relation is valid in the regime of a sufficiently large
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correction largely dominates.
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However, in practice, due to the residual higher-order terms, the coefficient $\alpha^{(m)}$ deviates slightly from unity.
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Using Eq.(\ref{eqx}) the estimated error on the CIPSI energy is calculated as
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Using Eq.~\eqref{eqx} the estimated error on the CIPSI energy is calculated as
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\begin{equation}
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E_{\text{CIPSI}}^{(m)} - E_{\text{FCI}}^{(m)}
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= \qty(E_\text{var}^{(m)}+E_{\text{rPT2}}^{(m)}) - E_{\text{FCI}}^{(m)}
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@ -311,11 +311,11 @@ state is given by
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\Delta E_{\text{FCI}}^{(m)}
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= \qty[ E_\text{var}^{(m)} + E_{\text{rPT2}} + \qty(\alpha^{(m)}-1) E_{\text{rPT2}} ]
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- \qty[ E_\text{var}^{(0)} + E_{\text{rPT2}} + \qty(\alpha^{(0)}-1) E_{\text{rPT2}} ]
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+ O\qty[{E_{\text{rPT2}}^2 }]
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+ \mathcal{O}\qty[{E_{\text{rPT2}}^2 }],
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\end{equation}
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which evidences that the error in $\Delta E_{\text{FCI}}^{(m)}$ can be expressed as $\qty(\alpha^{(m)}-\alpha^{(0)}) E_{\text{rPT2}} + O\qty[{E_{\text{rPT2}}^2}]$.
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which evidences that the error in $\Delta E_{\text{FCI}}^{(m)}$ can be expressed as $\qty(\alpha^{(m)}-\alpha^{(0)}) E_{\text{rPT2}} + \mathcal{O}\qty[{E_{\text{rPT2}}^2}]$.
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Now, for the largest systems considered here, $\qty|{E_{\text{rPT2}}}|$ can be as large as 2~eV and, thus,
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Now, for the largest systems considered here, $\abs{E_{\text{rPT2}}}$ can be as large as 2~eV and, thus,
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the accuracy of the excitation energy estimates strongly depends on our ability to compensate the errors in the calculations.
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Here, we greatly enhance the compensation of errors by making use of
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our selection procedure ensuring that the PT2 values of both states
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@ -325,12 +325,12 @@ E_{\text{rPT2}}^{(0)} \approx E_{\text{rPT2}}^{(m)}$, and
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by using a common set of state-averaged natural orbitals with equal weights for the ground and excited states.
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This last feature tends to make the values of $\alpha^{(0)}$ and $\alpha^{(m)}$ very close to each other, such that the error on the energy difference
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is decreased.
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In the ideal case where we would be able to fully correlate the CIPSI calculations for the ground- and excited-states, the fluctuations of
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In the ideal case where we would be able to fully correlate the CIPSI calculations associated with the ground and excited states, the fluctuations of
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$\Delta E_\text{CIPSI}^{(m)}(n)$ as a function of $n$ would completely vanish and the exact excitation energy would be obtained from the first CIPSI iterations.
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Quite remarkably, in practice, numerical experience shows that the fluctuations with respect to the extrapolated value $\Delta E_\text{FCI}^{(m)}$ are small,
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zero-centered, almost independent of $n$ when not too close iteration
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numbers are considered, and display a Gaussian-like distribution.
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In addition, the fluctuations are found to be (very weakly) dependent on the iteration number $n$ (see, Fig.\ref{fig2}), so
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In addition, as stated just above, the fluctuations are found to be (very weakly) dependent on the iteration number $n$ (see Fig.~\ref{fig:histo}), so
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this dependence will not significantly alter our results and will not be considered here.
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We thus introduce the following random variable
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\begin{equation}
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@ -348,7 +348,7 @@ A natural choice for $\sigma^2(n)$, playing here the role of a variance, is
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\begin{equation}
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\sigma^2(n) \propto \qty[E_{\text{rPT2}}^{(m)}(n)]^2 + \qty[E_{\text{rPT2}}^{(0)}(n)]^2,
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\end{equation}
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which vanishes in the large-$n$ limit as it should be.
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which vanishes in the large-$n$ limit as it should.
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%%% FIGURE 2 %%%
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\begin{figure}
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@ -357,19 +357,19 @@ which vanishes in the large-$n$ limit as it should be.
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\caption{Histogram of the random variable $X^{(m)}$ (see, text). About 200 values of the transition energies
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for the 13 five- and six-membered ring molecules, both for the singlet and triplet transitions and for a number of CIPSI iterations, are used.
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The number $M$ of iterations kept is chosen according to the statistical test presented in the text.}
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\label{fig2}
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\label{fig:histo}
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\end{figure}
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The histogram of $X^{(m)}$ resulting from the excitation energies
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obtained at different values of the CIPSI iterations $n$
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and for the 13 five- and six-membered ring molecules, both for the singlet and triplet transitions,
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is shown in Fig.\ref{fig2}. To avoid transient effects, only excitation energies at sufficiently large $n$ are retained in the data set.
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is shown in Fig.~\ref{fig:histo}. To avoid transient effects, only excitation energies at sufficiently large $n$ are retained in the data set.
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The criterion used to decide from which precise value of $n$ the data should be kept will be presented below. In our application, the total number
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of values employed to make the histogram is about 200. The dashed line of Fig.\ref{fig2} represents the best Gaussian fit
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of values employed to make the histogram is about 200. The dashed line of Fig.~\ref{fig:histo} represents the best Gaussian fit
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(in the sense of least-squares) reproducing the data.
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As seen, the distribution can be described by the Gaussian probability
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\begin{equation}
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P\qty[X^{(m)}] \propto e^{-\frac{{X^{(m)}}^2} {2{\sigma^{*}}^2}}
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P\qty[X^{(m)}] \propto \exp[-\frac{{X^{(m)}}^2} {2{\sigma^{*}}^2} ]
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\end{equation}
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where $\sigma^{*2}$ is some "universal" variance depending only
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on the way the correlated selection of both states is done, not on the molecule considered in our set.
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@ -380,7 +380,7 @@ $$\Delta E_\text{FCI}^{(m)} = \frac{ \sum_{n=1}^M \frac{\Delta E_\text{CIPSI}^{
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$$
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where $M$ is the number of data kept.
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Now, regarding the estimate of the error on $\Delta E_\text{FCI}^{(m)}$ some caution is required since, although the distribution is globally Gaussian-like
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(see Fig.\ref{fig2}) there exists
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(see Fig.~\ref{fig:histo}) there exists
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some significant departure from it and we need to take this feature into account.
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More precisely, we search for a confidence interval $\mathcal{I}$ such that the true value of the excitation energy $\Delta E_{\text{FCI}}^{(m)}$ lies within one standard deviation of $\Delta E_\text{CIPSI}^{(m)}$, i.e., $P\qty( \Delta E_{\text{FCI}}^{(m)} \in \qty[ \Delta E_\text{CIPSI}^{(m)} \pm \sigma ] \; \Big| \; \mathcal{G}) = 0.6827$.
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@ -1238,9 +1238,10 @@ MAE & & 0.22 & 0.16 & 0.22 & 0.11 & 0.12 & 0.05 & 0.04 & 0.02 & 0.20 & 0.22
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\end{threeparttable}
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\end{sidewaystable}
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%%% FIGURE 5 %%%
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\begin{figure}
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\centering
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\includegraphics[width=0.9\textwidth]{histograms}
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\includegraphics[width=0.9\textwidth]{fig5}
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\caption{Distribution of the error (in eV) in excitation energies (with respect to the TBE/aug-cc-pVTZ values) for various methods for the entire QUEST database considering only closed-shell compounds.
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Only the ``safe'' TBEs are considered (see Table \ref{tab:TBE}).
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See Table \ref{tab:stat} for the values of the corresponding statistical quantities.
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