antoine's full read
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@ -14,6 +14,7 @@
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\newcommand{\trash}[1]{\textcolor{red}{\sout{#1}}}
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\newcommand{\trashHB}[1]{\textcolor{orange}{\sout{#1}}}
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\newcommand{\antoine}[1]{\textcolor{cyan}{#1}}
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\newcommand{\trashantoine}[1]{\textcolor{cyan}{\sout{#1}}}
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\usepackage[
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colorlinks=true,
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@ -146,7 +147,7 @@ Each of these points is pedagogically illustrated using the Hubbard dimer at hal
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% SPIKE THE READER
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Perturbation theory isn't usually considered in the complex plane.
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Normally it is applied using real numbers as one of very few availabe tools for
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Normally it is applied using real numbers as one of very few available tools for
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describing realistic quantum systems where exact solutions of the Schr\"odinger equation are impossible \titou{to find?}.\cite{Dirac_1929}
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In particular, time-independent Rayleigh--Schr\"odinger perturbation theory\cite{RayleighBook,Schrodinger_1926}
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has emerged as an instrument of choice among the vast array of methods developed for this purpose.%
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@ -173,7 +174,7 @@ systematically improvable series largely remains an open challenge.
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% COMPLEX PLANE
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Our conventional view of electronic structure theory is centred around the Hermitian notion of quantised energy levels,
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where the different electronic states of a molecular are discrete and energetically ordered.
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where the different electronic states of a molecular \antoine{molecule or molecular system?} are discrete and energetically ordered.
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The lowest energy state defines the ground electronic state, while higher energy states
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represent electronic excited states.
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However, an entirely different perspective on quantisation can be found by analytically continuing
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@ -475,7 +476,7 @@ ultimately determines the convergence properties of the perturbation series.
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In the \trash{Hartree--Fock (HF)} \titou{HF} approximation, the many-electron wave function is approximated as a single Slater determinant $\Psi^{\text{HF}}(\vb{x}_1,\ldots,\vb{x}_\Ne)$, where $\vb{x} = (\sigma,\vb{r})$ is a composite vector gathering spin and spatial coordinates.
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This Slater determinant is defined as an antisymmetric combination of $\Ne$ (real-valued) occupied one-electron spin-orbitals $\phi_p(\vb{x})$, which are, by definition, eigenfunctions of the one-electron Fock operator
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\begin{equation}\label{eq:FockOp}
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\Hat{f}(\vb{x}) \phi_p(\vb{x}) = \qty[ \Hat{h}(\vb{x}) + \Hat{v}_\text{HF}(\vb{x}) ] \phi_p(\vb{x}) = \epsilon_p \phi_p(\vb{x}).
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\Hat{f}(\vb{x}) \phi_p(\vb{x}) = \qty[ \Hat{h}(\vb{x}) + \Hat{v}^{\antoine{\text{HF}}}(\vb{x}) ] \phi_p(\vb{x}) = \epsilon_p \phi_p(\vb{x}).
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\end{equation}
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Here the (one-electron) core Hamiltonian is
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\begin{equation}
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@ -636,7 +637,7 @@ Alternatively, the non-linear terms arising from the Coulomb and exchange operat
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be considered as a perturbation from the core Hamiltonian \eqref{eq:Hcore} by introducing the
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transformation $U \to \lambda\, U$, giving the parametrised Fock operator
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\begin{equation}
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\Hat{f}(\vb{x} ; \lambda) = \Hat{h}(\vb{x}) + \lambda\, \Hat{v}_\text{HF}(\vb{x}).
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\Hat{f}(\vb{x} ; \lambda) = \Hat{h}(\vb{x}) + \lambda\, \Hat{v}^{\antoine{\text{HF}}}(\vb{x}).
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\end{equation}
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The orbitals in the reference problem $\lambda=0$ correspond to the symmetry-pure eigenfunctions of the one-electron core
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Hamiltonian, while self-consistent solutions at $\lambda = 1$ represent the orbitals of the true HF solution.
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@ -884,7 +885,7 @@ In contrast, for $U = 4.5t$ one finds $\rc < 1$, and the RMP series becomes dive
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The corresponding Riemann surfaces for $U = 3.5\,t$ and $4.5\,t$ are shown in Figs.~\ref{subfig:RMP_3.5} and
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\ref{subfig:RMP_4.5}, respectively, with the single EP at $\lep$ (black dot) and the radius of convergence indicated
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by the vertical cylinder of unit radius.
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For the divergent case, the $\lep$ lies inside this cylinder of convergence, while in the convergent case $\lep$ lies
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For the divergent case, the $\lep$ \antoine{(\sout{the} $\lep$)} lies inside this cylinder of convergence, while in the convergent case $\lep$ lies
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outside this cylinder.
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In both cases, the EP connects the ground state with the doubly-excited state, and thus the convergence behaviour
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for the two states using the ground-state RHF orbitals is identical.
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@ -961,7 +962,7 @@ for larger $U/t$ as the radius of convergence becomes increasingly close to one
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As the UHF orbitals break the spin symmetry, new coupling terms emerge between the electronic states that
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cause fundamental changes to the structure of EPs in the complex $\lambda$-plane.
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For example, while the RMP energy shows only one EP between the ground state and
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the doubly-excited state (Fig.~\ref{fig:RMP}), the UMP energy has two EPs: one connecting the ground state with the
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the doubly-excited state (Fig.~\ref{fig:RMP}), the UMP energy has two (\antoine{pairs of}) EPs: one connecting the ground state with the
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singly-excited open-shell singlet, and the other connecting this single excitation to the
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doubly-excited second excitation (Fig.~\ref{fig:UMP}).
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This new ground-state EP always appears outside the unit cylinder and guarantees convergence of the ground-state energy.
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