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@ -1137,7 +1137,7 @@ This analysis highlights the importance of the primary critical point in control
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regardless of whether this point is inside or outside the complex unit circle. \cite{Handy_1985,Olsen_2000}
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%=======================================
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\subsection{The M{\o}ller--Plesset Critical Point}
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\subsection{M{\o}ller--Plesset Critical Point}
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\label{sec:MP_critical_point}
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%=======================================
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@ -1146,9 +1146,9 @@ In the early 2000's, Stillinger reconsidered the mathematical origin behind the
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sign alternation.\cite{Stillinger_2000}
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This type of convergence behaviour corresponds to Cremer and He's class B systems with closely spaced
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electron pairs and includes \ce{Ne}, \ce{HF}, \ce{F-}, and \ce{H2O}.\cite{Cremer_1996}
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Stillinger proposed that the divergence of these series arise from a dominant singularity
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Stillinger proposed that these series diverge due to a dominant singularity
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on the negative real $\lambda$ axis, corresponding to a multielectron autoionisation threshold.\cite{Stillinger_2000}
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To understand Stillinger's argument, consider the paramterised MP Hamiltonian in the form
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To understand Stillinger's argument, consider the parametrised MP Hamiltonian in the form
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\begin{multline}
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\label{eq:HamiltonianStillinger}
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\hH(\lambda) =
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@ -1164,7 +1164,7 @@ The mean-field potential $v^{\text{HF}}$ essentially represents a negatively cha
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controlled by the extent of the HF orbitals, usually located close to the nuclei.
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When $\lambda$ is negative, the mean-field potential becomes increasingly repulsive, while the explicit two-electron
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Coulomb interaction becomes attractive.
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There is therefore a critical value $\lc < 0$ where it becomes energetically favourable for the electrons
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There is therefore a negative critical value $\lc$ where it becomes energetically favourable for the electrons
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to dissociate and form a bound cluster at an infinite separation from the nuclei.\cite{Stillinger_2000}
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This autoionisation effect is closely related to the critial point for electron binding in two-electron
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atoms (see Ref.~\onlinecite{Baker_1971}).
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@ -1174,9 +1174,9 @@ processes.\cite{Sergeev_2005}
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% CLASSIFICATIONS BY GOODSOON AND SERGEEV
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To further develop the link between the critical point and types of MP convergence, Sergeev and Goodson investigated
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the relationship with the location of the dominant singularity that controls the radius of convergence.\cite{Goodson_2004}
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They demonstrated that the dominant singularity in class A corresponds to a dominant EP with a positive real component,
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with the magnitude of the imaginary component controlling the oscillations in the signs of the MP
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term.\cite{Goodson_2000a,Goodson_2000b}
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They demonstrated that the dominant singularity in class A systems corresponds to a dominant EP with a positive real component,
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with the magnitude of the imaginary component controlling the oscillations in the signs of successive MP
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terms.\cite{Goodson_2000a,Goodson_2000b}
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In contrast, class B systems correspond to a dominant singularity on the negative real $\lambda$ axis representing
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the MP critical point.
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The divergence of class B systems, which contain closely spaced electrons (\eg, \ce{F-}), can then be understood as the
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@ -1187,15 +1187,15 @@ and ii) $\beta$ singularities which have very small imaginary parts.\cite{Goodso
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% RELATIONSHIP TO BASIS SET SIZE
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The existence of the MP critical point can also explain why the divergence observed by Olsen \etal\ in the \ce{Ne} atom
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and \ce{HF} molecule occured when diffuse basis functions were included.\cite{Olsen_1996}
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and \ce{HF} molecule occurred when diffuse basis functions were included.\cite{Olsen_1996}
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Clearly diffuse basis functions are required for the electrons to dissociate from the nuclei, and indeed using
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only compact basis functions causes the critical point to disappear.
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While a finite basis can only predict complex-conjugate branch point singularities, the critical point is modelled
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by a cluster of sharp avoided crossings between the ground state and high-lying excited states.\cite{Sergeev_2005}
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Alternatively, Sergeev \etal\ demonstrated that the inclusion of a ``ghost'' atom also
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allows the formation of the critical point as the electrons form a bound cluster occuping the ghost atom orbitals.\cite{Sergeev_2005}
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This effect explains the origin of the divergence in the \ce{HF} molecule as the \ce{F} valence electrons jump to \ce{H}
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for a sufficiently negative $\lambda$ value.\cite{Sergeev_2005}
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allows the formation of the critical point as the electrons form a bound cluster occupying the ghost atom orbitals.\cite{Sergeev_2005}
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This effect explains the origin of the divergence in the \ce{HF} molecule as the fluorine valence electrons jump to hydrogen at
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a sufficiently negative $\lambda$ value.\cite{Sergeev_2005}
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Furthermore, the two-state model of Olsen \etal{}\cite{Olsen_2000} was simply too minimal to understand the complexity of
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divergences caused by the MP critical point.
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@ -1220,12 +1220,12 @@ divergences caused by the MP critical point.
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When a Hamiltonian is parametrised by a variable such as $\lambda$, the existence of abrupt changes in the
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eigenstates as a function of $\lambda$ indicate the presence of a zero-temperature quantum phase transition (QPT).%
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\cite{Heiss_1988,Heiss_2002,Borisov_2015,Sindelka_2017,CarrBook,Vojta_2003,SachdevBook,GilmoreBook}
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As an avoided crossing becomes increasingly sharp, the corresponding EPs move increasingly close to the real axis, eventually forming a critical point.
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Meanwhile, as an avoided crossing becomes increasingly sharp, the corresponding EPs move increasingly close to the real axis, eventually forming a critical point.
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The existence of an EP \emph{on} the real axis is therefore diagnostic of a QPT.\cite{Cejnar_2005, Cejnar_2007a}
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Since the MP critical point corresponds to a singularity on the real $\lambda$ axis, it can immediately be
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recognised a QPT within the perturbation theory approximation.
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recognised as a QPT within the perturbation theory approximation.
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However, a conventional QPT can only occur in the thermodynamic limit, which here is analogous to the complete
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basis set limit in our case.\cite{Kais_2006}
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basis set limit.\cite{Kais_2006}
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The MP critical point $\beta$ singularity in a finite basis must therefore be modelled by pairs of EPs
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that tend towards the real axis, exactly as described by Sergeev \etal\cite{Sergeev_2005}
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In contrast, $\alpha$ singularities correspond to large avoided crossings that are indicative of low-lying excited
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