goign through Hugh stuff and starting working on HF

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Pierre-Francois Loos 2020-11-17 13:31:03 +01:00
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@ -101,10 +101,12 @@
\newcommand{\bbR}{\mathbb{R}} \newcommand{\bbR}{\mathbb{R}}
\newcommand{\bbC}{\mathbb{C}} \newcommand{\bbC}{\mathbb{C}}
\newcommand{\Lup}{\text{L}^{\uparrow}} \newcommand{\Lup}{\mathcal{L}^{\uparrow}}
\newcommand{\Ldown}{\text{L}^{\downarrow}} \newcommand{\Ldown}{\mathcal{L}^{\downarrow}}
\newcommand{\Rup}{\text{R}^{\uparrow}} \newcommand{\Lsi}{\mathcal{L}^{\sigma}}
\newcommand{\Rdown}{\text{R}^{\downarrow}} \newcommand{\Rup}{\mathcal{R}^{\uparrow}}
\newcommand{\Rdown}{\mathcal{R}^{\downarrow}}
\newcommand{\Rsi}{\mathcal{R}^{\sigma}}
\newcommand{\LCPQ}{Laboratoire de Chimie et Physique Quantiques (UMR 5626), Universit\'e de Toulouse, CNRS, UPS, France.} \newcommand{\LCPQ}{Laboratoire de Chimie et Physique Quantiques (UMR 5626), Universit\'e de Toulouse, CNRS, UPS, France.}
@ -189,38 +191,32 @@ More importantly here, although EPs usually lie off the real axis, these singula
\subcaption{\label{subfig:FCI_cplx}} \subcaption{\label{subfig:FCI_cplx}}
\end{subfigure} \end{subfigure}
\caption{% \caption{%
\hugh{Exact energies for the Hubbard dimer ($U=4t$) as functions of $\lambda$ on the real axis (\subref{subfig:FCI_real}) and in the complex plane (\subref{subfig:FCI_cplx}). Exact energies for the Hubbard dimer ($U=4t$) as functions of $\lambda$ on the real axis (\subref{subfig:FCI_real}) and in the complex plane (\subref{subfig:FCI_cplx}).
Only the interacting closed-shell singlets are plotted in the complex plane, becoming degenerate at the EP (black dot).} Only the interacting closed-shell singlets are plotted in the complex plane, becoming degenerate at the EP (black dot).
\label{fig:FCI}} \label{fig:FCI}}
\end{figure*} \end{figure*}
\hugh{To illustrate the concepts discussed throughout this article, we will consider the symmetric Hubbard dimer at half filling, \ie\ with two opposite-spin fermions. To illustrate the concepts discussed throughout this article, we will consider the symmetric Hubbard dimer at half filling, \ie\ with two opposite-spin fermions.
Using the localised Wannier basis, the Hilbert space for this system comprises the four configurations Using the localised site basis, the Hilbert space for this system comprises the four configurations
\begin{equation} \begin{align}
\ket{\Lup \Ldown} \qquad \ket{\Lup\Rdown} \qquad \ket{\Rup\Ldown} \qquad \ket{\Rup\Rdown} & \ket{\Lup \Ldown} & & \ket{\Lup\Rdown} & & \ket{\Rup\Ldown} & & \ket{\Rup\Rdown}
\end{equation} \end{align}
%\begin{tabularx}{\linewidth}{YYYY} where $\Lsi$ ($\Rsi$) denotes an electron with spin $\sigma$ on the left (right) site.
%$\ket{\Lup \Ldown}$ & $\ket{\Lup\Rdown}$ & $\ket{\Rup\Ldown}$ & $\ket{\Rup\Rdown}$
%\\
%$\uddot \quad \vac$ & $\updot \quad \dwdot$ & $\dwdot \quad \updot$ & $\vac \quad \uddot$
%\end{tabularx}
where $\text{L}^{\sigma}$ ($\text{R}^{\sigma}$) denotes an electron with spin $\sigma$ on the left (right) site.
The exact Hamiltonian is then The exact Hamiltonian is then
\begin{equation} \begin{equation}
\label{eq:H_FCI} \label{eq:H_FCI}
\bH = \bH =
\begin{pmatrix} \begin{pmatrix}
U & - t & - t & 0 \\ U & - t & + t & 0 \\
- t & 0 & 0 & - t \\ - t & 0 & 0 & - t \\
- t & 0 & 0 & - t \\ + t & 0 & 0 & + t \\
0 & - t & - t & U \\ 0 & - t & + t & U \\
\end{pmatrix}, \end{pmatrix},
\end{equation} \end{equation}
where $t$ is the hopping parameter and $U$ is the on-site Coulomb repulsion.} where $t$ is the hopping parameter and $U$ is the on-site Coulomb repulsion.
We refer the interested reader to Refs.~\onlinecite{Carrascal_2015,Carrascal_2018} for more details about this system. We refer the interested reader to Refs.~\onlinecite{Carrascal_2015,Carrascal_2018} for more details about this system.
\hugh{%
To illustrate the formation of an exceptional points, we scale the off-diagonal coupling strength by introducing the complex parameter $\lambda$ through the transformation $t\rightarrow \lambda t$. To illustrate the formation of an exceptional points, we scale the off-diagonal coupling strength by introducing the complex parameter $\lambda$ through the transformation $t\rightarrow \lambda t$.
When $\lambda$ is real, the Hamiltonian~\eqref{eq:H_FCI} is Hermitian with the distinct (real-valued) eigenvalues When $\lambda$ is real, the Hamiltonian~\eqref{eq:H_FCI} is Hermitian with the distinct (real-valued) eigenvalues
\begin{subequations} \begin{subequations}
@ -233,10 +229,10 @@ E_{\text{T}} &= 0,
E_{\text{S}} &= U. E_{\text{S}} &= U.
\end{align} \end{align}
\end{subequations} \end{subequations}
While the open-shell triplet ($E_{\text{T}}$) and singlet ($E_{\text{S}}$) are independent of $\lambda$, the closed-shell singlet ground state ($E_{-}$) and doubly-excited state ($E_{+}$) couple strongly to form an avoided crossing at $\lambda=0$ (see Figure~\ref{subfig:FCI_real}). While the open-shell triplet ($E_{\text{T}}$) and singlet ($E_{\text{S}}$) are independent of $\lambda$, the closed-shell singlet ground state ($E_{-}$) and doubly-excited state ($E_{+}$) couple strongly to form an avoided crossing at $\lambda=0$ (see Fig.~\ref{subfig:FCI_real}).
At non-zero values of $U$ and $t$, these closed-shell singlets can only become degenerate at a pair of complex conjugate points in the complex $\lambda$ plane At non-zero values of $U$ and $t$, these closed-shell singlets can only become degenerate at a pair of complex conjugate points in the complex $\lambda$ plane
\begin{equation} \begin{equation}
\lambda_{\text{EP}} = \pm \frac{U}{4t} \i, \lambda_{\text{EP}} = \pm \i \frac{U}{4t},
\end{equation} \end{equation}
with energy with energy
\begin{equation} \begin{equation}
@ -247,9 +243,7 @@ These $\lambda$ values correspond to so-called EPs and connect the ground and ex
Crucially, the energy surface becomes non-analytic at $\lambda_{\text{EP}}$ and a square-root singularity forms with two branch cuts running along the imaginary axis from $\lambda_{\text{EP}}$ to $\pm \i \infty$ (see Fig.~\ref{subfig:FCI_cplx}). Crucially, the energy surface becomes non-analytic at $\lambda_{\text{EP}}$ and a square-root singularity forms with two branch cuts running along the imaginary axis from $\lambda_{\text{EP}}$ to $\pm \i \infty$ (see Fig.~\ref{subfig:FCI_cplx}).
On the real $\lambda$ axis, these EPs lead to the singlet avoided crossing at $\lambda = \Re(\lambda_{\text{EP}})$. On the real $\lambda$ axis, these EPs lead to the singlet avoided crossing at $\lambda = \Re(\lambda_{\text{EP}})$.
The ``shape'' of this avoided crossing is related to the magnitude of $\Im(\lambda_{\text{EP}})$, with smaller values giving a ``sharper'' interaction. The ``shape'' of this avoided crossing is related to the magnitude of $\Im(\lambda_{\text{EP}})$, with smaller values giving a ``sharper'' interaction.
}
\hugh{
Remarkably, the existence of these square-root singularities means that following a complex contour around an EP in the complex $\lambda$ plane will interconvert the closed-shell ground and excited states. Remarkably, the existence of these square-root singularities means that following a complex contour around an EP in the complex $\lambda$ plane will interconvert the closed-shell ground and excited states.
This behaviour can be seen by expanding the radicand in Eq.~\eqref{eq:singletE} as a Taylor series around $\lambda_{\text{EP}}$ to give This behaviour can be seen by expanding the radicand in Eq.~\eqref{eq:singletE} as a Taylor series around $\lambda_{\text{EP}}$ to give
\begin{equation} \begin{equation}
@ -259,7 +253,7 @@ Parametrising the complex contour as $\lambda(\theta) = \lambda_{\text{EP}} + R
\begin{equation} \begin{equation}
E_{\pm} \qty(\theta) \approx E_{\text{EP}} \pm \sqrt{32t^2 \lambda_{\text{EP}} R}\, \exp(\i \theta/2) E_{\pm} \qty(\theta) \approx E_{\text{EP}} \pm \sqrt{32t^2 \lambda_{\text{EP}} R}\, \exp(\i \theta/2)
\end{equation} \end{equation}
such that} such that
\begin{align} \begin{align}
E_{\pm}(2\pi) & = E_{\mp}(0), E_{\pm}(2\pi) & = E_{\mp}(0),
& &
@ -394,13 +388,28 @@ is the HF mean-field potential with
\begin{subequations} \begin{subequations}
\begin{gather} \begin{gather}
\label{eq:CoulOp} \label{eq:CoulOp}
J_i(\vb{x})\phi_p(\vb{x})=\qty[\int\dd\vb{x}'\phi_i(\vb{x}')\frac{1}{\abs{\vb{r} - \vb{r}'}}\phi_i(\vb{x}') ] \phi_p(\vb{x}) J_i(\vb{x})\phi_p(\vb{x})=\qty[\int \phi_i(\vb{x}')\frac{1}{\abs{\vb{r} - \vb{r}'}}\phi_i(\vb{x}') \dd\vb{x}' ] \phi_p(\vb{x})
\\ \\
\label{eq:ExcOp} \label{eq:ExcOp}
K_i(\vb{x})\phi_p(\vb{x})=\qty[\int\dd\vb{x}'\phi_i(\vb{x}')\frac{1}{\abs{\vb{r} - \vb{r}'}}\phi_p(\vb{x}') ] \phi_i(\vb{x}) K_i(\vb{x})\phi_p(\vb{x})=\qty[\int \phi_i(\vb{x}')\frac{1}{\abs{\vb{r} - \vb{r}'}}\phi_p(\vb{x}') \dd\vb{x}'] \phi_i(\vb{x})
\end{gather} \end{gather}
\end{subequations} \end{subequations}
being the Coulomb and exchange operators (respectively) in the spin-orbital basis. \cite{SzaboBook} being the Coulomb and exchange operators (respectively) in the spin-orbital basis. \cite{SzaboBook}
The HF energy is then defined as
\begin{equation}
\label{eq:E_HF}
E_\text{HF} = \sum_i h_i + \frac{1}{2} \sum_{ij} \qty( J_{ij} - K_{ij} )
\end{equation}
with
\begin{subequations}
\begin{gather}
h_i = \int \phi_i(\vb{x}) h(\vb{x}) \phi_i(\vb{x}) \dd\vb{x}
\\
J_{ij} = \int \phi_i(\vb{x}) J_j(\vb{x}) \phi_i(\vb{x}) \dd\vb{x}
\\
K_{ij} = \int \phi_i(\vb{x}) K_j(\vb{x}) \phi_i(\vb{x}) \dd\vb{x}
\end{gather}
\end{subequations}
If the spatial part of the spin-orbitals are restricted to be the same for spin-up and spin-down electrons, one talks about restricted HF (RHF) theory, whereas if one does not enforce this constrain it leads to the so-called unrestricted HF (UHF) theory. If the spatial part of the spin-orbitals are restricted to be the same for spin-up and spin-down electrons, one talks about restricted HF (RHF) theory, whereas if one does not enforce this constrain it leads to the so-called unrestricted HF (UHF) theory.
From hereon, $i$ and $j$ are occupied orbitals, $a$ and $b$ are unoccupied (or virtual) orbitals, while $p$, $q$, $r$, and $s$ indicate arbitrary orbitals. From hereon, $i$ and $j$ are occupied orbitals, $a$ and $b$ are unoccupied (or virtual) orbitals, while $p$, $q$, $r$, and $s$ indicate arbitrary orbitals.
@ -409,17 +418,17 @@ Rather than solving Eq.~\eqref{eq:SchrEq}, HF theory uses the variational princi
\hH^{\text{HF}} = \sum_{i} f(\vb{x}_i). \hH^{\text{HF}} = \sum_{i} f(\vb{x}_i).
\end{equation} \end{equation}
Coming back to the Hubbard dimer, the HF energy is Coming back to the Hubbard dimer, the HF energy is [see Eq.~\eqref{eq:E_HF}]
\begin{equation} \begin{equation}
E_\text{HF} = -t \qty[ \sin \theta_\alpha + \sin \theta_\beta ] + \frac{U}{2} \qty[ 1 + \cos \theta_\alpha \cos \theta_\beta ] E_\text{HF} = -t \qty[ \sin \theta_\alpha + \sin \theta_\beta ] + \frac{U}{2} \qty[ 1 + \cos \theta_\alpha \cos \theta_\beta ]
\end{equation} \end{equation}
where where
\begin{align} \begin{align}
\psi_{1\sigma} & = \cos(\frac{\theta_\sigma}{2}) s_1 - \sin(\frac{\theta_\sigma}{2})s_2 \psi_{1\sigma} & = \cos(\frac{\theta_\sigma}{2}) \Lsi - \sin(\frac{\theta_\sigma}{2}) \Rsi
\\ \\
\psi_{2\sigma} & = \sin(\frac{\theta_\sigma}{2}) s_1 + \cos(\frac{\theta_\sigma}{2})s_2 \psi_{2\sigma} & = \sin(\frac{\theta_\sigma}{2}) \Lsi + \cos(\frac{\theta_\sigma}{2}) \Rsi
\end{align} \end{align}
are the one-electron molecule orbitals for the spin-$\sigma$ electrons and the angles which makes the energy stationnary, \ie, $\pdv*{E_\text{HF}}{\theta_\sigma} = 0$ are given by
\begin{align} \begin{align}
\theta_\alpha & = \arctan (-\frac{\sqrt{U^2 - 4t^2}}{U},\frac{2t}{U}) \theta_\alpha & = \arctan (-\frac{\sqrt{U^2 - 4t^2}}{U},\frac{2t}{U})
\\ \\