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@ -1295,11 +1295,11 @@ orbitals [see Eq.~\eqref{eq:RHF_orbs}] with $\theta_{\alpha}^{\text{RHF}} = \the
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%\begin{equation}
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% E_\text{HF}(0, 0) = \frac{1}{2} (2 U - 4 \epsilon).
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%\end{equation}
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With this representation, the parametrised RMP Hamiltonian becomes
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With this representation, the parametrised \hugh{atomic} RMP Hamiltonian becomes
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\begin{widetext}
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\begin{equation}
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\label{eq:H_RMP}
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\bH_\text{RMP}\qty(\lambda) =
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\hugh{\bH_\text{atom}\qty(\lambda)} =
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\begin{pmatrix}
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2(U-\epsilon) - \lambda U & -\lambda t & -\lambda t & 0 \\
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-\lambda t & (U-\epsilon) - \lambda U & 0 & -\lambda t \\
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@ -1337,7 +1337,8 @@ In contrast, smaller $\epsilon$ gives a weaker attraction to the atomic site,
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representing strong screening of the nuclear attraction by core and valence electrons,
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and again a less negative $\lambda$ is required for ionisation to occur.
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Both of these factors are common in atoms on the right-hand side of the periodic table, \eg\ \ce{F},
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\ce{O}, \ce{Ne}, and thus molecules containing these atoms are often class $\beta$ systems with
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\ce{O}, \ce{Ne}.
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Molecules containing these atoms are therefore often class $\beta$ systems with
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a divergent RMP series due to the MP critical point. \cite{Goodson_2004,Sergeev_2006}
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% EXACT VERSUS APPROXIMATE
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@ -1384,10 +1385,9 @@ For $\lambda>1$, the HF potential becomes an attractive component in Stillinger'
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Hamiltonian displayed in Eq.~\eqref{eq:HamiltonianStillinger}, while the explicit electron-electron interaction
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becomes increasingly repulsive.
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Critical points along the positive real $\lambda$ axis for closed-shell molecules then represent
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points where the two-electron repulsion overcomes the attractive HF potential and an electron
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are successively expelled from the molecule.\cite{Sergeev_2006}
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points where the two-electron repulsion overcomes the attractive HF potential and a single electron dissociates from the molecule.\cite{Sergeev_2006}
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Symmetry-breaking in the UMP reference creates different HF potentials for the spin-up and spin-down electrons.
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In contrast, symmetry-breaking in the UMP reference creates different HF potentials for the spin-up and spin-down electrons.
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Consider the reference UHF solution where the spin-up and spin-down electrons are localised on the left and
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right sites respectively.
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The spin-up HF potential will then be a repulsive interaction from the spin-down electron
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@ -1413,7 +1413,7 @@ occurs exactly at $\lambda = 1$.
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In this limit, the ground-state EPs approach the real axis (Fig.~\ref{subfig:ump_ep_to_cp}) and the avoided
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crossing creates a gradient discontinuity in the ground-state energy (dashed lines in Fig.~\ref{subfig:ump_cp}).
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We therefore find that, in the strong correlation limit, the symmetry-broken ground-state EP becomes
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a new type of MP critical and represents a QPT in the perturbation approximation.
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a new type of MP critical point and represents a QPT in the perturbation approximation.
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Furthermore, this argument explains why the dominant UMP singularity lies so close, but always outside, the
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radius of convergence (see Fig.~\ref{fig:RadConv}).
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