diff --git a/Manuscript/EPAWTFT.tex b/Manuscript/EPAWTFT.tex index 719e479..362f668 100644 --- a/Manuscript/EPAWTFT.tex +++ b/Manuscript/EPAWTFT.tex @@ -972,15 +972,11 @@ exact correlation energy $\Delta E$ using terms up to MP6\cite{Cremer_1996} &= \Emp^{(2)} + \Emp^{(3)} + \qty(\Emp^{(4)} + \Emp^{(5)}) \exp(\Emp^{(6)} / \Emp^{(5)}). \end{align} \end{subequations} -%As one can only compute the first terms of the MP series, a smart way of getting more accurate results is to use extrapolation formula, \ie, estimating the limit of the series with only few terms. -%Cremer and He proved that using specific extrapolation formulas of the MP series for class A and class B systems improves the precision of the results compared to the formula used without resorting to classes. \cite{Cremer_1996} These class-specific formulas reduced the mean absolute error from the FCI correlation energy by a factor of four compared to previous class-independent extrapolations, highlighting how one can leverage a deeper understanding of MP convergence to improve estimates of the correlation energy at lower computational costs. -In Section~\ref{sec:Resummation}, we consider more advanced extrapolation routines that take account of EPs in the complex $\lambda$-plane. -%The mean absolute deviation taking the FCI correlation energies as reference is $0.3$ millihartree with the class-specific formula whereas the deviation increases to 12 millihartree using the general formula. -%Even if there were still shaded areas in their analysis and that their classification was incomplete, the work of Ref.~\onlinecite{Cremer_1996} clearly evidenced that understanding the origin of the different modes of convergence could potentially lead to a more rationalised use of MP perturbation theory and, hence, to more accurate correlation energy estimates. +In Sec.~\ref{sec:Resummation}, we consider more advanced extrapolation routines that take account of EPs in the complex $\lambda$-plane. In the late 90's, Olsen \etal\ discovered an even more concerning behaviour of the MP series. \cite{Olsen_1996} They showed that the series could be divergent even in systems that were considered to be well understood, @@ -1007,7 +1003,7 @@ Using their observations in Ref.~\onlinecite{Olsen_1996}, Olsen and collaborator a simple method that performs a scan of the real axis to detect the avoided crossing responsible for the dominant singularities in the complex plane. \cite{Olsen_2000} By modelling this avoided crossing using a two-state Hamiltonian, one can obtain an approximation for -the dominant singularities as the EPs of the $2\times2$ matrix +the dominant singularities as the EPs of the two-state matrix \begin{equation} \label{eq:Olsen_2x2} \underbrace{\mqty(\alpha & \delta \\ \delta & \beta )}_{\bH} @@ -1027,10 +1023,8 @@ These intruder-state effects are analogous to the EP that dictates the convergen the RMP series for the Hubbard dimer (Fig.~\ref{fig:RMP}). Furthermore, the authors demonstrated that the divergence for \ce{Ne} is due to a back-door intruder state that arise when the ground state undergoes sharp avoided crossings with highly diffuse excited states. -%They used their two-state model on this avoided crossings and the model was actually predicting the divergence of the series. -%They concluded that the divergence of the series was due to the interaction with a highly diffuse excited state. This divergence is related to a more fundamental critical point in the MP energy surface that we will -discuss in Section~\ref{sec:MP_critical_point}. +discuss in Sec.~\ref{sec:MP_critical_point}. Finally, Ref.~\onlinecite{Olsen_1996} proved that the extrapolation formulas of Cremer and He \cite{Cremer_1996} are not mathematically motivated when considering the complex singularities causing the divergence, and therefore @@ -1053,7 +1047,6 @@ according to a so-called ``archetype'' that defines the overall ``shape'' of the For Hermitian Hamiltonians, these archetypes can be subdivided into five classes (zigzag, interspersed zigzag, triadic, ripples, and geometric), while two additional archetypes (zigzag-geometric and convex-geometric) are observed in non-Hermitian Hamiltonians. -%Other features characterising the convergence behaviour of a perturbation method are its rate of convergence, its length of recurring period, and its sign pattern. The geometric archetype appears to be the most common for MP expansions,\cite{Olsen_2019} but the ripples archetype corresponds to some of the early examples of MP convergence. \cite{Handy_1985,Lepetit_1988,Leininger_2000}